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NAME ____________________________________________________________________________PER _______ DATE ___________________
GENETICS REVIEW
We all know that children tend to resemble their parents in appearance. Parents and children generally have
similar eye color, hair texture, height and other characteristics because children inherit genes that control
specific characteristics from their parents. When we talk about genes being inherited from one generation to
the next, we are really talking about how the gene-carrying chromosomes behave during meiosis and
fertilization.
1. Explain why each cell in a baby's body has two copies of each gene.
Inheritance of Albinism
To learn more about how genetic traits are inherited, we will
consider a specific example -- the gene that controls whether
or not a person can produce the pigment melanin which
contributes to the color of skin, eyes and hair. Some people
have the hereditary condition, albinism; they are not able to
produce melanin and have little or no pigment in their skin and
hair. Two different versions of the same gene are called alleles.
One allele of this gene codes for melanin production and normally
pigmented skin and hair; it is symbolized by A. Another allele
of this gene codes for albinism; it is symbolized by a.
We'll analyze inheritance for the case where each parent has one A allele and one a allele (i.e. both parents are
Aa).
2. What different combinations of A and/or a alleles would you expect to observe in the children of these
parents?
Aa x aa
aa x aa
Aa x Aa
Biologists use the Punnett Square to answer this type of question. The figure below shows more details than a
typical Punnett Square. It shows that, as a result of meiosis, half of the mother's eggs will have a chromosome
which carries the A allele, and half will have a chromosome with the a allele. Similarly, half of the father's sperm
will have an A allele, and half will have the a allele. Typically, Punnett squares exclude much of the explanatory
material we have included in the above Punnett square.
A
3. Fill in the Punnett Square below.
A
4. What fraction (not ratio) of this couple's children would you expect to be AA?
1/4
5. What fraction (not ratio) of this couple's children would you expect to be Aa?
2/4 or 1/2
6. What fraction (not ratio) of this couple's children would you expect to be aa?
1/4
a
a
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The children who have AA alleles will have normal pigmentation, and the children who have aa alleles will have
albinism. Often, one allele in a heterozygous pair of alleles is dominant and the other allele is recessive.
7. Which trait is dominant?
The normal pigmentation trait is dominant
8. Which trait is recessive?
The albinism trait is recessive
9. What pigmentation will be observed for the children who have Aa alleles?
Children with Aa alleles will show normal pigmentation
10. Someone who is Aa is said to be homozygous or heterozygous. Circle one.
The genotype refers to the genetic makeup of an individual. The phenotype refers to the observable physical
and physiological characteristics of an individual. Give an example of two individuals who have the same
phenotype, but different genotypes for the albinism gene.
11. Explain how two individuals with the same phenotype can have different genotypes.
If someone is homozygous dominant (DD) they will show normal pigmentation. If
someone is heterozygous (Dd) they will show normal pigmentation as well because the
dominant trait masks the recessive trait.
Biologists frequently express the fractions of different genotypes or phenotypes as ratios. For example, the
genotype fractions ¼ AA, ½ Aa, ¼ aa are frequently expressed as a 1:2:1 ratio.
12. For the corresponding phenotypes, the fraction with normal pigment is _____ and the fraction with albinism
is _____, so the corresponding ratio is ____________.
Suppose a father has aa alleles and a mother has Aa alleles. Use a Punnett Square to determine what fraction of
this couple's children would be expected to have albinism.
Ratio of normal to albino:
13.__________________
Coin Genetics
The way genes behave can be easily simulated using a two-sided coin, where tails represents the recessive allele
that controls pigment production (a), and heads represents the dominant allele (A). Each result of tossing the
coin represents the allele present in an egg or sperm produced by a parent who is heterozygous (Aa).
In order to simulate the types of offspring produced by these parents, you need to find another person with
whom to “mate”. Each person then tosses their coin to make a pair of genes (one person’s toss represents the
gene donated by the sperm, the other person’s toss the gene donated by the egg). Each pair of genes represents
the genotype of an individual offspring.
A. Find someone with whom to “mate”.
B. Each of you will toss your coin to make pairs of genes. Make 8 children in each round; i.e. record the results of
8 tosses of the coins. Now write down in the row labeled “Round 1” how many of each of the 3 possible
combinations, AA, Aa, or aa, you obtained.
C. Now make 8 more children with another partner by tossing the coins and record the genotypes in the “Round
2” row.
D. Do this one more time and record the results in the rows for Rounds 3.
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AA
Aa
aa
Round 1
Round 2
Round 3
Total
Fractions
E. Add up your results to determine the total number of AA, Aa, and aa combinations in the children produced
by your coin tosses. Calculate the fractions of these children who have each of the three genotypes. Compare the
results for these children (produced by your coin toss matings between two heterozygous parents) with the
predictions from the Punnett Square shown on page 2.
13. What are the predicted fractions from page 2?
14. Are the fractions of each genotype in these children similar to the predicted fractions?
15. Is there any difference between your results and the predictions?
In many cases, the results for a family of four children will not match the predictions of the Punnett Square.
Random variation in which particular sperm fertilizes which particular egg explains why the children in the
individual families may differ considerably from the predictions based on a Punnett Square. However, results
for a large number of children from multiple pairs of parents with the same genetic makeup are usually close to
the predictions of the Punnett Square.
Genetics of Sex Determination and Sex-Linked Disorders
The principles described in the previous section apply also to the gene which controls whether a person will be
a male or a female. This gene is located on the Y chromosome in humans. It is called SRY, which stands for sexdetermining region on the Y chromosome. If a zygote has a Y chromosome with the SRY gene, the zygote will
develop testes and male anatomy. If a zygote does not have a Y chromosome with the SRY gene, the zygote will
develop ovaries and female anatomy.
As you probably know, males have an X and a Y chromosome (XY), whereas females have two X chromosomes
(XX). During meiosis in a female, the two X-chromosomes separate, so each egg has a single X-chromosome. In
males, even though the X and the Y-chromosomes are very different, they can nevertheless pair with each other
and separate from each other during meiosis. This means that males produce two kinds of sperm; half have an X
chromosome and half have a Y chromosome.
16. What will be the sex of a child produced when an egg is fertilized by a
sperm that has a Y chromosome?
17. Draw a Punnett Square which shows the inheritance of the sex
chromosomes.
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18. Based on this Punnett Square, what fraction of children would you expect to be male?
19. If a mother's first child is a son, is the next child necessarily a daughter? Why or why not?
No. Punnett squares only show possible outcomes.
Sex-linked disorders are carried on the X chromosome. Because the male only has sex-linked genes on the X
chromosome, there is nothing on theY chromosome to mask the disorder on the X chromosome. So if a male has
even 1 copy of the disordered gene, he suffers from the disorder. A female can carry 1 disordered gene on one X
chromosome but still have a normal gene to mask it on the other X chromosome. A "carrier female" usually
doesn't suffer from the disorder, she just carries it and can pass it on .... usually to her SONS as you'll soon see!
Hemophilia is a blood-coagulation disorder in which the blood fails to clot normally because of a deficiency or
abnormality of one of the clotting factor proteins. Hemophilia is a recessive trait associated with the Xchromosome. The allele for hemophilia is Xh. Assume a woman is XHXh and a man is XHY and they have children.
20. Fill in the Punnett Square below.
21. Does the woman have the disorder?
No, but she is a carrier of the disorder.
22. Will the couple have any children with the disorder?
There is a 25% chance their child will have the disorder.
23. What is the gender of the children with the disorder?
The child who would get the disorder would be a male.
24. Why is a male more likely than a female to get the disorder?
The disorder is a sex-linked trait, meaning it's a recessive disorder carried on the xchromosome. Males only have one X, making it easier for them to inherit the disorder.
Genetics of Sickle cell anemia and codominance
For some genes, neither allele is completely dominant, so a heterozygous individual differs from either type of
homozygous individual. (A heterozygous individual has two different alleles for the same gene, whereas a
homozygous individual has two identical alleles for the same gene. For example individuals that are either AA
or aa are homozygous, whereas individuals that are Aa are heterozygous.)
The gene for hemoglobin is an example. One allele codes
for normal hemoglobin, while another allele codes for
altered hemoglobin, called sickle cell hemoglobin. When a
person is homozygous for this sickle cell allele, this causes
a serious disease called sickle cell anemia. The sickle cell
hemoglobin tends to cause the red blood cells to assume a
sickle shape, in contrast to the normal disk-shaped red
blood cell shown on the left in the figure below. Oxygen
cannot be transported to tissues by the sickle blood cells.
Individuals with sickle cell anemia can also suffer from
fatigue, breathlessness, rapid heart rate, delayed growth
and puberty, susceptibility to infections ulcers on the
lower legs (in adolescents and adults) jaundice, attacks of
abdominal pain, weakness, joint pain, fever, and vomiting.
25. What problems might be caused by the sickle-shaped
red blood cells?
Oxygen and nutrients can't be transported through the body. Symptoms include fatigue,
breathlessness, rapid heart rate, delayed grow the and puberty, etc.
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A person who is heterozygous for the sickle cell allele often has very few or no symptoms of sickle cell
anemia. In addition, people who are heterozygous for the sickle cell allele are more resistant to malaria, an
infection of the red blood cells which is transmitted by mosquitoes in many tropical countries. Sickle cells
cannot become infected. Thus, in areas where malaria is widespread, people who are heterozygous for the sickle
cell allele are less likely to become ill and die from malaria or sickle cell anemia because they make both normal
red blood cells and sickle cells. This is an example of codominance. Because of this advantage, the sickle cell
allele became relatively common in regions like West Africa where malaria is common. Since African-Americans
are descended from populations in which the sickle cell allele was relatively common, African-Americans have
relatively high rates of the sickle cell allele (approximately 8% are heterozygous for this allele and 0.16% are
homozygous).
Suppose that a person who is heterozygous for the sickle cell allele (NS) marries a person who is also
heterozygous for this allele (NS).
26. Fill in the Punnett Square to show the expected genetic makeup
of their children, and then answer the following questions.
27. What fraction of their children will suffer from sickle cell
anemia?
1/4
28. What fraction of their children will inherit the sickle cell allele,
but will not suffer from sickle cell anemia? (These children will
be resistant to malaria.)
1/2
29. Who is more likely to be resistant to malaria? Explain.
The heterozygous carriers are more resistant. They show little to no symptoms but still
produce sickle shaped cells, making it harder for malaria to spread.
Genetics of incomplete dominance and codominance
Incomplete dominance is when both alleles of a heterozygote influence the phenotype. The phenotype is usually
intermediate between the two homozygous phenotypes. The situation in which a heterozygote shows a
phenotype somewhere (but not exactly half-way) intermediate between the corresponding homozygote
phenotypes. In northeast Kansas there is a creature know as a wildcat. It comes in three colors, blue, red, and
purple. This trait is an example of incomplete dominance. A homozygous (BB) individual is blue, a homozygous
(bb) individual is red, and a heterozygous (Bb) individual is purple.
30. What are the results of crossing a blue wildcat with a red one?
The result would be purple offspring.
31. What would be the genotypes and phenotypes?
BB - Blue
Bb - Purple
bb - red
32. If the wildcat fur color was an example of a codominant trait, what would the phenotypes of the cross be?
Blue with red spots or red with blue spots
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Genetics of Multiple Allele Inheritance and Bloodtyping
The ABO system is called a multiple allele system for there are
more than two possible allele pairs for the locus. The
individual's blood type is determined by which combination of
alleles he/she has. There are four possible blood types in order
from most common to most rare: O, A, B and AB. Human blood
is separated into different classifications because of the varying
proteins (antigens) contained in each blood type's red blood
cells. These proteins are there to identify whether or not the
blood in the individual's body is it's own and not something the
immunity system should destroy. The protein's structure is
controlled by three alleles; i, IA and IB.
33. What are the possible genotypes for someone with type A
blood?
34. Fill in a Punnett square for the cross between a
heterozygous type A individual and a type O individual.
35. Can two people with type AB blood have a child with
type O blood? Explain.
Neither parent has the recessive allele (i) to pass on to their
children.
Puppy Genetics
Imagine this microscopic drama. A sperm cell from a male dog fuses with an egg cell from a female dog. Each
dog’s gamete carries 39 chromosomes. The zygote that results from the fusion of the gametes contains 78
chromosomes- one set of 39 chromosomes from each parent. One of the zygote’s chromosomes are show
below.
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Each chromosome of the homologous pair contains alleles for the same traits. But one chromosome may have a
dominant allele and the other a recessive allele. Use the drawings and the table to answer the questions.
36. What is the puppy’s genotype for coat pattern?
37. Does the female (mother) have a spotted coat? Explain.
Yes, the mom has a spotted coat. For a spotted coat, the genotype could be AA or Aa. The
mom passed down 'A' to her puppy, so we know that she had to have either of the dominant
genotypes.
38. Can you tell if the male (father) would have a spotted coat? Explain.
No. The father passed 'a' down to his puppy. The dad could have had Aa or aa for his
genotype, showing two different phenotypes.
39. What will be the texture of the puppy’s coat?
Wiry
40. Is it possible for the puppy to have puppies (when he is old enough) with straight hair? Explain.
Yes. The puppy carries the recessive allele 't'. If the puppy mates with another dog that had
the recessive allele, they may have puppies with long hair.
41. For which traits is the puppy heterozygous?
The pup is heterozygous for all but curliness.
42. Explain why you cannot completely describe the puppy’s parents even though you can accurately describe
the puppy. Be sure to cite the data.
Parents can give 2 different alleles depending on what their genotype is. The picture only tells
us one of each parents' alleles so we have no way of knowing what the other allele could be.
43. Would knowing the grandparents’ genotypes help you to determine the parents’ genotypes? Explain.
Yes. If we knew the grandparents' genotypes, we could make a Punnett square to determine the
possible genotypes and phenotypes of their offspring.