How are chickens affected by light?

How are chickens affected by light?

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When I was young, I was told never to put a fluorescent light in a hen house because it continuously turns on and off. Extra lighting is used, as far as I know, to 'extend' a chicken's day. Human eyes don't have the refresh frequency to notice that, but there are animals who have a frame-rate of up to 200 Hz (so they actually see the lights turn off and back on again). I was told chickens are among those animals and that they'd stop laying eggs when continuously exposed to interrupted lighting.

However, my neighbour has fluorescent lighting in his hen house. So either the above is bogus, or I have my facts mixed up. So, how (if at all) are chickens affected by light?

It seems like they are not affected by fluorescent light frequency. I did not find anything about their visual sampling rate. Their hearing is between 0-200Hz with an average of 86Hz so I guess the visual sampling rate is under this, but that's just a guess.

We conclude that at the illumination levels used in this experiment, the hens did not perceive the flicker of low-frequency light or they perceived it but did not find it aversive. Low-frequency fluorescent light does not appear to adversely affect the welfare of hens.

1996 - Laying hens do not have a preference for high-frequency versus low-frequency compact fluorescent light sources

It concludes that there is no evidence that fluorescent or high pressure sodium lighting, irrespective of intensity or spectral distribution, has any consistent detrimental effect on growth, food utilization, reproductive performance, mortality, behaviour or live bird quality in either domestic fowl or turkeys, nor in the egg production of geese.

1998 - Responses of domestic poultry to various light sources

A monochromatic (LED) light can be more beneficial according to this:

A significant reduction in egg production was observed in all 880nm groups; no differences in egg production and quality were found in the other groups. Feed consumption was significantly lower by 7% in all 0.01 W/m2 groups. We suggest that an important reduction in rearing costs of laying hens may be obtained by using this system.

1998 - New monochromatic light source for laying hens

Study of Chicken Teeth Sheds Light on Evolution

A new study shows that chickens, which don't have teeth, still have the genes that make them, and in special cases, those genes can be switched back on. Scientists now think that as animals evolve, they lose the ability to turn those genes on at the right time during development &mdash not the genes themselves.

It is a well-known fact of science that chickens don't have teeth. So well known that it's been incorporated into everyday speech, rare as hen's teeth, the saying goes. But facts have a way of shifting in science. Researchers at the University of Wisconsin have found a mutant strain of chicken that does appear to have teeth. And as NPR's Joe Palca reports, the finding sheds light on how evolution works.

The notion that chickens can't make teeth has been around a good long while. But in 1821 a French zoologist named Geoffroy Saint-Hilaire tried challenging it. In an article entitled (French Spoken), On the Teeth of Birds, he described sharp brown structures beneath the beaks of parrots he was studying. Saint-Hilaire said these structures were nascent teeth. C'est impossible, asserted his contemporary Georges Leopold Chretien Frederic Dagobert, Baron Cuvier--everyone knows birds can't make teeth. Cuvier said the sharp brown structures might be sprouting feathers sprouting in the wrong place. But teeth? No. And so the dogma endured, until recently.

Enter biologist John Fallon of the University of Wisconsin and his graduate student Matthew Harris. Harris was working in the lab late one night when he saw strange structures beneath the beak of a chick embryo he was studying. Fallon says Harris showed him the structures the next day.

Mr. JOHN FALLON (Biologist, University of Wisconsin): They looked like teeth.

PALCA: And to Fallon they didn't look just like any teeth. As he reports in the latest issue of the journal Current Biology, they looked like alligator teeth. Alligator baby teeth, mind you, not the big choppers they get as adults, but alligator teeth all the same. Fallon says that may sound peculiar, but it really isn't that strange because birds descend from a class of reptiles, reptiles with teeth, known as Arcosaurs.

Mr. FALLON: The closest living relative of birds in that lineage are the crocodilians, and alligator being one.

PALCA: Now, you may be saying to yourself, wait a minute, how come no one spotted these teeth before? Good question. The reason is Fallon was working with a strain of chickens called Talpid 2 mutants. These mutants grow abnormally and don't live long enough to hatch, but they do help scientists understand the causes of birth defects. Fallon says something about the Talpid 2 mutation allows a feature lost 60 million years ago, teeth in this case, to pop out again.

And it turns out that hens' teeth aren't the only lost features that can pop back up among animals. Michael Levine is a biologist at the University of California Berkeley. He says under the right circumstances snakes can grow the legs their ancestors once had. And cavefish can grow the eyes their ancestors once had. It's as if the animals have a genetic memory of their ancient past, and Levine says in a way they do.

Mr. MICHAEL LEVINE (Biologist, University of California, Berkeley): The genes for teeth are still in the bird genomes. The genes for limbs are still in the snake genomes. And the genes for eyes are still in the cavefish genomes. So what do we make of this? The genes have not been lost, they've been retained despite the loss of the structures and the reason is simple. The genes are used for other purposes.

PALCA: In the case of birds, the same genes that once made teeth are now used to make feathers. And Levine says this realization that genes are retained has helped scientists understand how evolution works. Levine says nearly all animals have the same set of genes.

Mr. LEVINE: What's different about them is how they use these genes. Where and when these genes are turned on and off is radically different from animal to animal.

PALCA: And it's the intricate dance of genes interacting with each other that changes during the course of evolution. And there's one funny coincidence about the chicken teeth story. Remember that guy who said birds couldn't make teeth? Georges Leopold Chretien Frederic Dagobert, Baron Cuvier? Well, it turns out that he trained a scientist, who trained a scientist, who trained a scientist, who trained a scientist, who trained a scientist, who trained John Fallon, the man who showed that Baron Cuvier was wrong about chicken teeth. An example of evolution in science, perhaps. Joe Palca, NPR News, Washington.

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Photoperiodism, the functional or behavioral response of an organism to changes of duration in daily, seasonal, or yearly cycles of light and darkness. Photoperiodic reactions can be reasonably predicted, but temperature, nutrition, and other environmental factors also modify an organism’s response.

In animals, the regular activities of migration, reproduction, and the changing of coats or plumage can be induced out of season by artificially altering daylight. Birds, for example, have migrated north in the winter after having been exposed to reversed seasonal lighting in laboratories. The manipulation of a specific stimulating period of darkness, which is required by each species for every phase of the migratory process, is an important factor in photoperiodism.

Light Affects Mood and Learning through Distinct Retina-Brain Pathways

Light exerts a range of powerful biological effects beyond image vision, including mood and learning regulation. While the source of photic information affecting mood and cognitive functions is well established, viz. intrinsically photosensitive retinal ganglion cells (ipRGCs), the central mediators are unknown. Here, we reveal that the direct effects of light on learning and mood utilize distinct ipRGC output streams. ipRGCs that project to the suprachiasmatic nucleus (SCN) mediate the effects of light on learning, independently of the SCN's pacemaker function. Mood regulation by light, on the other hand, requires an SCN-independent pathway linking ipRGCs to a previously unrecognized thalamic region, termed perihabenular nucleus (PHb). The PHb is integrated in a distinctive circuitry with mood-regulating centers and is both necessary and sufficient for driving the effects of light on affective behavior. Together, these results provide new insights into the neural basis required for light to influence mood and learning.

Keywords: aberrant light cycle circadian rhythms ipRGCs learning mood perihabenular nucleus suprachiasmatic nucleus ventromedial prefrontal cortex.

Nationwide chicken shortage affects local business

CARBONDALE (WSIL) -- If you're a fan of chicken wings, some of your favorite restaurants might be experiencing a shortage.

Many of us spent the last year ordering take-out from chicken restaurants, but now some executives say the high demand, slow production and other factors are causing chicken supplies to run low.

News 3 spoke to Harold's Chicken in Carbondale, a business that just opened in December 2020.

Store Manager Tachia Smith says they've experienced a shortage of their own for the past two and a half months.

She says they've been low on chicken wings and chicken breasts and have had to push customers to try their seafood to make up for the demand.

"We've been experiencing the chicken shortage first-hand here with different vendors that we've tried to find different locations that we've been running low on that's been completely out. If we wanted to order more, we couldn't order because they have X-amount per person to get, you know?" she said.

She also says the vendors that they buy chicken from have increased their prices about 10% and she says they may have to increase the price of some of their meals.

The National Chicken Council says the weather is partly to blame for the shortage. The winter storm in Texas caused a halt for some production companies. They say supply should catch up with demand soon.

Maya Skinner

Maya Skinner is a Multimedia Journalist at News 3 WSIL. Maya joined the team in 2019 and graduated from the University of Missouri-Columbia in 2018 with a Bachelor’s in Communication with a focus in Mass Media.


A hen can lay only one egg in a day and will have some days when it does not lay an egg at all. The reasons for this laying schedule relate to the hen reproductive system. A hen’s body begins forming an egg shortly after the previous egg is laid, and it takes 26 hours for an egg to form fully. So a hen will lay later and later each day. Because a hen’s reproductive system is sensitive to light exposure, eventually the hen will lay too late in a day for its body to begin forming a new egg. The hen will then skip a day or more before laying again. See the related article discussing the reproductive tract of a chicken for more information on the specifics of egg production.

Also, hens in a flock do not all begin to lay on exactly the same day, nor do they continue laying for the same length of time. Figure 1 shows a typical egg production curve for a flock. The flock comes into production quickly, peaks, and then slowly reduces the level of production.

Figure 1. Typical egg production curve and egg weight values for egg-laying flocks

The length of time that a flock will produce eggs varies as well. Many home flocks produce eggs on and off for three to four years. Each year, the level of egg production is lower than the previous year. Also, egg size increases and shell quality decreases each year.

Both the number of eggs you can get from a flock and the number of years a flock will produce eggs depend on several variables, including the following factors:

  • Breed
  • Management of pullets prior to lay
  • Light management
  • Nutrition
  • Space allowances


Some commercial breeds of chickens have been developed specifically for egg production. The commercial White Leghorn is used in large egg production complexes, but these birds typically do not produce well in home flocks. They are simply too flighty. Moreover, they lay white-shelled eggs. People purchasing eggs from small flocks often prefer to buy brown-shelled eggs, even though no nutritional differences exist between brown-shelled eggs and white-shelled eggs.

Breeding companies also have developed commercial layers for brown-shelled egg production, with some bred specifically for pasture poultry production. In addition, many hatcheries sell what are called sex-link crosses. These specific crosses allow the hatchery to sex the chicks at hatch based on feather color. As a result, the number of sexing errors is reduced, so you are less likely to get an unwanted rooster.

Some people like having a flock composed of different breeds. Such a flock can produce eggs having a selection of shell colors. Many dual-purpose breeds, such as Plymouth Rocks and Rhode Island Reds, lay eggs with light brown shells. Maran hens lay eggs with dark, chocolate-colored shells, which have become popular lately. The Araucana is a South American breed that has feather tufts around the face and no tail and lays eggs having light blue shells. By crossing Araucanas with other breeds, breeders have produced “Easter Egger” hens that lay eggs with light blue, green, or pink shells. The chickens produced from these crosses have beards and muffs rather than the tufts seen on Araucanas, and they have tails. If bred to the purebred standards, such a cross will result in an Ameraucana, which lays eggs having blue-green shells.

Obviously, you can choose from several breeds. When making your decision about which breed or breeds to raise, keep in mind that commercial-type hens may give you a higher level of production initially, but other breeds tend to lay for more years. For additional assistance in deciding which breed to choose, see the related article on which chicken breed is best for a small or backyard flock .

Pullet management

It is important to manage pullets correctly, especially in the areas of nutrition and light management, because correct management will affect the level and quality of egg production once the birds start to lay. If the pullets come into production too early, they may have problems with prolapse , which can cause health problems across the flock. Also, the hens may lay smaller eggs throughout the production cycle.

When raising pullets from day-old chicks, brood the chicks as you would any other type of chick. See the related article on brooding poultry hatchlings for information about the basic care of chicks. For future laying flocks, keep in mind that light management is important from brooding through all laying periods.

If you purchase pullets ready-to-lay, you should ask how the pullets were raised with regard to nutrition and light management so that you can adjust your subsequent management of the flock accordingly. For example, you may have to delay light stimulation if the hens are too small.

Light management for year-round production

Chickens are called long-season breeders, meaning that they come into production as days become longer. That is, they start producing eggs when there are more hours of light per day. Typically, day-old chicks are kept on 23 to 24 hours of light per day for the first few days to make sure that they are able to find food and water, especially water. After that time period, you should reduce the number of hours of light per day. If you are raising the birds indoors, you can give them just 8 hours of light per day. If you are exposing them to outdoor conditions, you are limited by the number of hours of light per day in your area, of course. When the pullets are ready to start laying, slowly increase the light exposure until they are exposed to about 14 hours of light per day. This exposure should stimulate the flock to come into lay. To keep the flock in lay year-round, you will need to maintain a schedule of at least 14 hours of light per day. You can increase the amount of light slowly to 16 hours per day late in the egg production cycle to help keep the flock in production. For most flock owners, this strategy involves providing supplemental lighting. Using a light with a stop/start timer, you can cause the light to come on early in the morning before sunrise and in the evening before sunset to ensure that the length of light exposure for the flock totals 14 to 16 hours. Also, you can get a light sensor so that the light bulb does not come on when natural daylight is available. By using such a device, you minimize your electricity use. The supplemental light you provide does not have to be overly bright. A typical 60-watt incandescent light bulb works fine for a small laying flock. For a discussion of other light choices, watch the recording of the webinar Lighting for Small and Backyard Flocks by Dr. Michael Darre from the University of Connecticut.


Chickens of any type and age require a complete, balanced diet. Feed mills assemble the available ingredients in combinations that provide all the nutrients needed by a flock in one package. Some producers mix complete feeds with cheaper scratch grains, but doing so dilutes the levels of nutrients the chickens are receiving, and nutrient deficiencies can occur. Nutrient deficiencies can adversely affect the growth of pullets and the level of production of hens.

It is also important to feed the specific feed tailored for the type and age of the chickens you have. For example, do not feed a “meat-maker” type diet to growing pullets or laying hens as it will not meet their nutritional needs. Likewise, do not feed a layer diet to growing chickens. The diet of a laying hen is high in calcium, which is needed for the production of eggshells. This level of calcium, however, is harmful to non-laying chickens.

Some hens have a higher need for calcium than others. It is always good to have an additional source of calcium available. Oystershell, usually available in feed stores, is an excellent calcium supplement for a laying flock.

Space allowances

To produce effectively, laying hens must have adequate space. The amount of floor space required by a flock depends on the size of the chickens (which is related to the breed of chicken chosen) and the type of housing used. A minimum of 1.5 square feet per hen is recommended, with 2 square feet per hen being the most commonly used space allowance. Larger allowances are required for some of the larger breeds.

To make use of the entire housing facility, you can incorporate perches. The hens will sleep on the perches at night, keeping them off the floor. The use of perches also helps concentrate much of the manure in a single location for easier cleaning of the poultry house. Moreover, chickens have a desire to perch, so providing for this behavior contributes to animal welfare. For more information, read the related article on perches .

If you provide outdoor space for your chickens, the amount of outdoor space needed depends on the quality of the space. If your goal is to maintain a pasture, you will require more area than you would need if simply providing outdoor access for a small backyard flock. An allowance of 2 square feet per hen typically is recommended for simple outdoor access. If you do provide your flock with outdoor access, be aware of predator possibilities from both the ground and the air, and provide the hens with the protection they require.

Part I: Identifying Food Dyes in Candies

Many foods, drugs and cosmetics are artificially colored with federally approved food dyes (FD & C dyes). These dyes include Red 40, Red 3, Yellow 5, Yellow 6, Blue 1, and Blue 2. Since each dye has an identifiable absorption spectrum and peak, a spectrophotometer may be used to identify the types of FD & C dye used in a product.

Pigments may be extracted from foods and drinks that contain one or more of these dyes. An absorption spectrum of that extract can then determine what dyes are in that food or drink by comparing the peaks of maximum absorbance with information in the table below. If the absorption spectrum of a food extract has a peak at 630 nm and one at 428 nm, you can assume the food contains both Blue #1 and Yellow #5. The following table gives the wavelength of peak absorbance for each of these dyes.

Table 1. Wavelength of Maximum Absorbance of Commonly Used FD & C Dyes



A. Extracting Dye from Candy (Your Instructor will do this for you)

You will need one test tube and one cuvette for each color to be tested. Measure 4 mL water into one tube. Place 2-4 candies of the same color in a test tube with the water. Gently swirl, and wait one minute. After, pour approximately 1 mL of liquid into a microcentrifuge tube. Spin the microcentrifuge tube at max speed for 60 seconds. Make sure the centrifuge is balanced before spinning. Transfer the clear liquid (supernatant) into a cuvette. Make sure to leave behind the particulates (pellet).

Figure 4. Curve of absorbance

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My hen always likes to greet me personally, (I truly think it is her attempt to get the first mealy worms,) but anyway, she got so excited when she saw me coming she ran out of her nesting box and laid her egg on my foot, while leaning against me for her petting. It cracked me up and not the egg.

Eggs are completely clean when they come out. The only reason poop gets on them if is the coop is too dirty or her feet are not clean. Let your chickens walk around in a little pan of water once in a while they love it. anon954296 May 31, 2014

Thanks for the concise article. Very informative. anon924829 January 8, 2014

I forgot how much light chickens need to lay eggs. Also, usually after 11 or 12, months they will stop laying during molting, when they grow all new feathers, and it usually takes about a month for them to start laying again. After molting they will produce larger eggs, but maybe not every day. anon349860 September 30, 2013

It is really great to read this blog for it has complete ideas and techniques as well as giving helpful images that helped me understand the whole idea of hatching eggs. I wonder if they can give idea also on how to make an incubator. Thanks!

One of my chickens was away from the nest and I notice that she was running toward the nest with her back end kind of in a swat, and out came an egg. She was still running, trying to make it to her nest. So funny anon322655 February 28, 2013

Not so yummy when you actually take the time to think about the origin of the egg - from a chicken's uterus. No thanks! bluedolphin December 19, 2012

@anon49031-- I have seen chickens lay eggs closely many times. Not that I enjoy watching them do that, but I have to help my dad with raising chickens at the farm.

Anyway, I don't think it's painful. I don't meant to gross anyone out here, but the way they act while laying the egg is similar to a bowel movement. Just some exertion, the opening expands and whoop, the egg pops out. It's really easier and faster than what people think.

I know some hens make a lot of noise while doing it, but it's not from pain. If the chicken is acting like she's in pain while laying eggs, there might be something else going on. burcidi December 18, 2012

@anon284874-- I'm not an expert on this but I thought that they can start laying eggs before six months.

How are you keeping your chickens? Are they seeing sunlight? Don't forget they need to see light to lay eggs.

Also, don't be surprised if they just lay one egg per day and if the eggs are very small at first. Younger chickens tend to lay smaller eggs and the eggs will get larger over time. Older chickens will produce less eggs and might even stop laying eggs after a certain age.

They also tend to lay less eggs in winter since the day is shorter and they won't get as much light. fify December 17, 2012

I feel like chickens are being cheated by humans. They go through so much to lay eggs every day and then we just take it and eat it.

Does laying eggs hurt them? It must right? It's like they're giving birth every day. I wonder why they lay eggs so often when other animals don't. anon284979 August 13, 2012

I have a chicken that is losing weight. I purchased her six weeks ago. I have wormed her but I think it's too late. Her crop seems empty all the time. Any ideas, please? The other three girls are fine. --Bob anon284874 August 12, 2012

I have six chickens I bought. I just read they start laying at six months old, and they are almost there. What is my next step to start getting some eggs? Help a poor Oklahoma girl out! anon283287 August 3, 2012

No, a rooster is not needed for an egg to be laid. A chicken will lay eggs regardless. The only reason a rooster would be needed is if one wanted to hatch the eggs (either through a brooding chicken, or an incubator). anon278205 July 5, 2012

Do chickens act different when it's time to start to lay eggs? Do they go off and sit away from the others? --Bob

very interesting stuff. But does anyone know why chicken lay eggs at all?(except for reproductive reasons). anon158644 March 8, 2011

thank you very much, you helped me in my project.

Since there is no umbilical, how do the embryos oxygenate? If they breathe, are they breathing air or liquid? Great article, thanks. anon138082 December 30, 2010

Can you please advise if chickens have a uterus?

This article says they do, but others I have read say they don't. Thanks. anon105482 18 hours ago

My husband told me an egg comes out of the same opening as the feces. I told him no way! I was very surprised and grossed out to learn he was correct. Thanks for the info and ending our argument on this subject. anon87319 May 29, 2010

wow. this was educational and informative enough! thanks so much. anon63195 January 31, 2010

Thanks for the info. you helped me with my science fair project! anon55383 December 7, 2009

Its very informative. I learned something new today about the egg-laying process. it was for our science class. The kids had so many questions about all of this and this article had all the answers thank you. anon49031 October 16, 2009

My chickens just laid there first egg, it sounded very painfull. is it? Even saw some blood on the shell. Great article about the egg cycle and how to keep them laying anon48514 October 13, 2009

I had two debates over the "chicken vs the egg". Through this article I was able to get both answers. anon43217 August 26, 2009

good information. i was really curious! anon42047 August 18, 2009

Thanks for the information. regards --keith anon40107 August 6, 2009

this is a very interesting article. i want to know further that how do chickens give birth to their child, i mean what is the process of brooding? anon16231 August 1, 2008

could a chicken that has never come in contact with a rooster still lay eggs? and is it pleasure or pain? why do they let out a distinct sound? anon16111 July 29, 2008

Very informative, have been around chickens for years and had no knowledge of the cycle. Thank you for sharing.

Problem 3

Flower position, stem length, and seed shape were three characters that Mendel studied. Each is controlled by an independently assorting gene and has dominant and recessive expression as follows:

Character Dominant Recessive
Flower position Axial (A ) Terminal (a )
Stem length Tall (T ) Dwarf (t )
Seed shape Round (R ) Wrinkled (r)

If a plant that is heterozygous for all three characters were allowed to self-fertilize, what proportion of the offspring would be expected to be as follows: (Note – use the rules of probability (and show your work) instead of huge Punnett squares)

a) homozygous for the three dominant traits

AATTRR = 1/4 x 1/4 x 1/4 = 1/64

b) homozygous for the three recessive traits

aattrr = 1/4 x 1/4 x 1/4 = 1/64

c) heterozygous (assumed for each trait)

AaTtRr = 1/2 x 1/2 x 1/2 = 1/8

d) homozygous for axial and tall, heterozygous for seed shape

AATTRr = 1/4 x 1/4 x 1/2 = 1/32

Problem 4 A black guinea pig crossed with an albino guinea pig produced 12 black offspring. When the albino was crossed with a second one, 7 blacks and 5 albinos were obtained.

What is the best explanation for this genetic situation?

Black is dominant over white

Write genotypes for the parents, gametes, and offspring.

Parent’s genotypes = BB (black) x bb (white)
gametes = B b
F1 offspring = all Bb

Parent’s genotypes = Bb (black) x bb (white)
gametes = B or b b
F1 offspring = Bb or bb

There should be 50% black to 50% white offspring in this cross.

Problem 5 In sesame plants, the one-pod condition (P ) is dominant to the three-pod condition (p ), and normal leaf (L ) is dominant to wrinkled leaf (l) . Pod type and leaf type are inherited independently. Determinine the genotypes for the two parents for all possible matings producing the following offspring:

a. 318 one-pod normal, 98 one-pod wrinkled

Parental genotypes: PPLl x PPLl or PpLl x PPLl

b. 323 three-pod normal, 106 three-pod wrinkled

Parental genotypes: ppLl x ppLl

Parental genotypes: PPLL x PpLL or PPLl x PPLL or PPLL x PpLl etc (nine possible genotypes).

d. 150 one-pod normal, 147 one-pod wrinkled, 51 three-pod normal, 48 three-pod wrinkled. (a 3: 3: 1: 1 ratio)

Parental genotypes: PpLl x Ppll (see below for details)

3 One-pod wrinkled (PPll , Ppll , Ppll)

1 Three-pod normal (ppLl)

1 Three-pod wrinked (ppll)

e. 223 one-pod normal, 72 one-pod wrinkled, 76 three-pod normal, 27 three-pod wrinkled (a 9: 3: 3: 1 ratio)

Parental genotypes: PpLl x PpLl

Problem 6 A man with group A blood marries a woman with group B blood. Their child has group O blood. What are the genotypes of these individuals?

First Child = OO (or ii)

What other genotypes and in what frequencies, would you expect in offspring from this marriage?

Examine the Punnett square to determine the other genotypes possible.

Problem 7 Color pattern in a species of duck is determined by one gene with three alleles. Alleles H and I are codominant, and allele i is recessive to both. Note: this situation is similar to the ABO blood system.

How many phenotypes are possible in a flock of ducks that contains all the possible combinations of these three alleles?

As in the ABO blood system 4 phenotypes are possible in this case:

Genotype Phenotype
HH, Hi (H)
II, Ii (I)
ii (i)

Problem 8 Phenylketonuria (PKU) is an inherited disease caused by a recessive allele. If a woman and her husband are both carriers, what is the probability of each of the following?

Under these circumstances assume the following Punnett square to be true.

a. all three of their children will be of normal phenotype

b. one or more of the three children will have the disease (x)

All three have x 2 out of 3 has x 1 out of 3 has x
+ + =
x x o o o x
3 Combinations x o x o x o
o x x x o o
+ 3(3/4 x 1/4 x 1/4) + 3(3/4 x 3/4 x 1/4) =

Note: the probability of the disease (x) = 1/4 & the probability of being normal (o) = 3/4

c. all three children will have the disease

d. at least one child out of three will be phenotypically normal

(Note: Remember that the probabilities of all possible outcomes always add up to 1)

Problem 9 The genotype of F1 individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent assortment of these four genes, what are the probabilities that F2 offspring would have the following genotypes?

a. aabbccdd = x x x = 1/256

b. AaBbCcDd = x x x = 1/16

c. AABBCCDD = x x x = 1/256

d. AaBBccDd = x x x = 1/64

e. AaBBCCdd = x x x = 1/128

Just remember that the probability of a heterozygote (Xx) = 2/4 or 1/2 and the probability of a homozygote XX or xx = 1/4

Problem 10 In 1981, a stray black cat with unusual rounded curled-back ears was adopted by a family in California. Hundreds of descendants of the cat have since been born, and cat fanciers hope to develop the “curl” cat into a show breed. Suppose you owned the first curl cat and wanted to develop a true breeding variety.

How would you determine whether the curl allele is dominant or recessive?

Mate the stray to a non-curl cat. If any offspring have the “curl” trait it is likely to be dominant. If the mutation is recessive, then on ly non-curl offspring will result.

How would you select for true-breeding cats?

You know that cats are true-breeding when curl crossed with curl matings produce only curl offspring.

How would you know they are true-breeding?

A pure-bred “curl cat” is homozygous.

  1. If the trait is recessive any inividual with the “curl” condition is homozygous recessive.
  2. If the trait is dominant you can determine if the individual in question is true breeding (CC) or heterozygous (Cc) with a test cross (to a homozygous recessive individual).

Problem 11 What is the probability that each of the following pairs of parents will produce the indicated offspring (assume independent assortment of all gene pairs?

a. AABBCC x aabbcc —-> AaBbCc

b. AABbCc x AaBbCc —–> AAbbCC

c. AaBbCc x AaBbCc —–> AaBbCc

d. aaBbCC x AABbcc —-> AaBbCc

Problem 12 Karen and Steve each have a sibling with sickle-cell disease. Neither Karen, Steve, nor any of their parents has the disease, and none of them has been tested to reveal sickle-cell trait. Based on this incomplete information, calculate the probability that if this couple should have another child, the child will have sickle-cell anemia.