Genetics Review

Objectives 1-5

Objective 1 - Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype.

• Define genotype

• Define phenotype

Answer• Genotype –Actual physical DNA

make up of an organism. 2) The genetic constitution of an individual, including alleles carried for a particular trait. Compare to phenotype. (i.e. A pea with a round phenotype could have a genotype of RR or Rr).

• Phenotype - Physical and behavioral characteristics of an organism. 2) An individual's observable form. Compare to genotype. (i.e. A pea with a round phenotype could have a genotype of RR or Rr).

• Define homozygous –

• Define heterozygous -

Answer• Homozygous - Having

identical alleles for a trait. 2) Carrying two copies of the same allele. Compare to heterozygous. (i.e. A homozygous plant could have the genotype RR or rr).

• Heterozygous - Having different alleles for a trait. 2) Carrying two different alleles. Compare to homozygous. (i.e. The heterozygous plant has the genotype Rr).

A test cross is used to determine if the genotype of a plant displaying the dominant phenotype is homozygous or heterozygous.

You always cross your unknown with a pure recessive (displays the recessive trait)

If the unknown is homozygous dominant, all of the offspring of the test cross have the __________ phenotype.

If the unknown is heterozygous, half of the offspring will have the __________ phenotype.

Answer

• dominant, recessive• Dominant is expressed if it is present, AA or

Aa• Recessive is expressed only if the dominant is

not present , aa

Objective 2 - Use a Punnett square to predict the results of a monohybrid and a dihybrid cross and state the phenotypic and genotypic ratios of the F2 generation.

1. Monohybrid Cross: look at one trait

In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?

Answer¾One fourth of the offspring will be homozygous dominant (SS), one half will be heterozygous (Ss), and one fourth will be homozygous recessive (ss).

Question 2

2. Monohybrid Cross

A phenotypic ratio of 3:1 in the offspring of a mating of two organisms heterozygous for a single trait is expected. How does this occur?

During what process does this occur?

Answer

• The alleles segregate during meiosis. Mendel first proposed that alleles segregate from one another during the formation of gametes.

Question 3Dihybrid Cross: look at two traits

True-breeding pea plants with spherical seeds were crossed with true-breeding plants with dented seeds. (Spherical seeds are the dominant characteristic.) Mendel collected the seeds from this cross, grew F1-generation plants, let them self-pollinate to form a second generation, and analyzed the seeds of the resulting F2 generation. The results that he obtained, and that you would predict for this experiment are: (give both the F1 and F2 generations)

True Breeding or pure = homozygousHybrid = heterozygous

All the F1 and 3/4 of the F2 generation seeds were spherical. All of the F1 plants were true hybrids with a phenotype of Ss. The recessive trait reappears in the F2 generation.

Question 4

• A genetic cross between two F1-hybrid pea plants for spherical seeds will yield what percent spherical-seeded plants in the F2 generation? (Recall, spherical-shaped seeds are dominant over dented seeds.)

Answer

• 75%

Dihybrid cross

• A pea plant is heterozygous for both seed shape and seed color. S is the allele for the dominant, spherical shape characteristic; s is the allele for the recessive, dented shape characteristic. Y is the allele for the dominant, yellow color characteristic; y is the allele for the recessive, green color characteristic. What will be the distribution of these two alleles in this plant's gametes?

Answer25% of gametes are SY; 25% of gametes are Sy;25% of gametes are sY; 25% of gametes are sy.

Alleles of different genes are assorted independently of each other during the formation of gametes.

• Which of the following genetic crosses would be predicted to give a phenotypic ratio of 9:3:3:1?

A. SSYY x ssyy B. SsYY x SSYy C. SsYy x SsYy D. SSyy x ssYY E. ssYY x ssyy

Answer

C. SsYy x SsYy

• What is the expected phenotypic ratio of the progeny of a SsYy x ssyy test cross?

AnswerSsYy, ssYy, Ssyy, ssyy are predicted to occur in a ratio of 1:1:1:1.

• In a dihybrid cross, AaBb x AaBb, what fraction of the offspring will be homozygous for both recessive traits?

Answer

• 1/4 of the gametes of each parent will be ab. The fraction of the offspring homozygous for both recessive traits will be 1/4 times 1/4, or 1/16.

Objective 3 - Explain how the phenotypic expression of the heterozygote is affected by complete dominance, incomplete dominance, and co-dominance.

• With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with an intermediate phenotype (3rd phenotype) that is a blending of the parental traits.

A cross between a blue blahblah bird & a white blahblah bird produces offspring that are silver. The color of blahblah birds is determined by just two alleles.

a) What are the genotypes of the parent blahblah birds in the original cross?

b) What is/are the genotype(s) of the silver offspring?

c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?

Answer

a) What are the genotypes of the parent blahblah birds in the original cross? Since there are only 2 alleles & three phenotypes (blue, white, & silver), we must be dealing with incomplete dominance. So the blue parent is homozygous blue (BB) & the white parent is homozygous white (bb).

b) What is/are the genotype(s) of the silver offspring?

The silver offspring are hybrids (Bb), one blue allele & one white allele, neither one dominating the other. Instead, we get a blending of blue & white, i.e. silver.

c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?

silver x silver = Bb x Bb.

25% (1/4) of the offspring are homozygous white (bb), 25% (1/4) are homozygous blue (BB), & 50% (2/4) are hybrid & therefore have the silver phenotype.

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.

1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. (Roan is a result of brown and white hairs being closely located to give the appearance of reddish)

2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with brown fur?

3. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritence does this illustrate? b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

Answer1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

Step #1 --- recognize that "roan" is a codominance trait. Homozygous white = WW, & roan = RW (a hybrid cow).

So our cross is WW x RW & the punnett square should look something like what you see here. The results:

2/4 offspring (50%) will be roan (RW), & 50% will be white (WW).

2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with brown fur? Well, the only way to have brown fur is to be homozygous brown (RR). In order to get that genotype in all the offspring both parents must be "RR". A parent with one or more "W" alleles will cause the inheritence of roan fur in some offspring. Only RR x RR gives you 100% RR. RR x RW would produce 50% roan, 50% brown, RW x RW produces 25% brown, 50% roan & 25% white, WW x RW would produce 50% roan, 50% white, & WW x RR would produce 100% roan (RW).

3. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritence does this illustrate? Codominance, two phenotypes together at the same time. b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat? Tabby cats are the hybrids (because they have both colors) & a black cat must be homozygous black. So the cross for this problem is BB (black) x BT (tabby

The results show that 50% of the offspring will be BB (black) & 50% will be tabby (BT). So to answer the question, 0% of the kittens will be tan.

Blood Type Problem

• In humans the blood groups are produced by various combinations of three alleles IA, IB, and i. Blood type A is caused by either IA IA or IA i; type B by IB IB or IB i; type AB by IA IB; and type O by i i. Suppose a child is of blood type A and the mother is of type 0. What type or types may the father belong to?

Answer

• Since the mother can only provide alleles for O type blood (i), the father must provide the allele for blood type A (IA). Three genotypes can provide the IA allele: IA IA (blood type A), IA i (blood type A), or IA IB (blood type AB). So the father must be either blood type A or blood type AB. The child (with blood type A) must be heterozygous, IA i (remember the O allele, i, is recessive to both the A and the B alleles).

Blood Type Problem

• Suppose a father of blood type A and a mother of blood type B have a child of type O. What blood types are possible in their subsequent children?

Answer• Remember, because type O

blood results from the homozygous recessive genotype (i i ), the only way to produce a type O child is if both parents provide an O allele (i ).

• Since the father has blood type A, he must be heterozygous (IA i ).

• Similarly, since the mother has blood type B, she must be heterozygous as well, but with the B and O alleles (IB i ).

• Construct a Punnett square diagram to determine the possible offspring of these two parents:

• Based upon these results, we can see that these parents may produce offspring with the following blood type phenotypes: AB, A, B, and O.

• Suppose a father and mother claim they have been given the wrong baby at the hospital. Both parents are blood type A. The baby they have been given is blood type O. What evidence bearing on this case does this fact have?

Answer• Note that in this problem, all we know is the parental

phenotypes, not their genotypes. It is possible that both parents are heterozygous (IA i ). In which case, it would be possible to produce a type O child 25% of the time (see Punnett square diagram below).

• The baby may indeed belong to these parents. Other genetic tests would be required to know for sure one way or another.

Objective 4 - Explain the inheritance of sex-linked traits and disorders.

• Hemophilia in humans is due to an X-chromosome mutation. What will be the results of mating between a normal (non-carrier) female and a hemophilac male?

Answer

• All sons are normal • All daughters are

carriers

• A human female "carrier" who is heterozygous for the recessive, sex-linked trait causing red-green color blindness (or alternatively, hemophilia), marries a normal male. What proportion of their male progeny will have red-green color blindness

Answer

• Half the sons would be expected to inherit the allele from their mother and be afflicted because they are hemizygous. Half the daughters would be carriers like their mothers.

• Women have sex chromosomes of XX, and men have sex chromosomes of XY.

• Which of a man's grandparents could not be the source of any of the genes on his Y-chromosome?

A. Father's Mother. B. Mother's Father. C. Father's Father. D. Mother's Mother, Mother's Father, and Father's Mother. E. Mother's Mother.

Answer

D. Mother's Mother, Mother's Father, and Father's Mother. The Y chromosome is inherited solely from father to son in each generation.

• Women have sex chromosomes of XX, and men have sex chromosomes of XY.

• Which of a women's grandparents could not be the source of any of the genes on either of her X-chromosomes?

A. Mother's Father. B. Father's Mother. C. Mother's Mother. D. Father's Father. E. Mother's Mother and Mother's Father.

Answer

D. Father's Father. The father's father contributes only the Y chromosome to his sons, and subsequently to his grandsons.

• The alleles for eye color and for body color are on the X chromosome of Drosophila, but not on the Y. Red eye color (w+) is dominant to white eye color (w), and tan body color (y+) is dominant to yellow body color (y).

• What is the genotype of a yellow-bodied, red-eyed female who is homozygous for eye color?

Answer

• The alleles for eye color and for body color are on the X chromosome of Drosophila, but not on the Y. Red eye color (w+) is dominant to white eye color (w), and tan body color (y+ ) is dominant to yellow body color (y). What is the genotype of a tan-bodied, white-eyed male?

Answer

• What offspring would you expect from a cross between the female Drosophila described in problem 1 (red eyes and a yellow body, homozygous recessive for the yellow body color allele and homozygous dominant for the eye color allele) and the male described in problem 2 (hemizygous for both the recessive (white) eye color allele and dominant (tan) body color allele?) A reminder that the alleles for eye color and for body color are on the X chromosome of Drosophila, but not on the Y. Red eye color (w+) is dominant to white eye color (w), and tan body color (y+) is dominant to yellow body color (y).

Answer

Objective 5 - Explain why dominant alleles do not necessarily mean that the allele is more common in a population.• The trait that we see mostly when we look around at people

is the prevalent trait. The next few slides give examples of some dominant and recessive traits in the human population.

As a class determine which of these traits are the most prevalent in the classroom

Thumbs• Straight thumbs (dominant trait) can be seen as nearly a

straight line and may contain a slight arch when viewed from the side as in the illustrations.

• Curved thumbs (recessive trait) can be seen as part of a circle.

PinkyBent pinky (dominant trait) vs. Straight pinky (recessive trait):

1. Hold your hands together as if you are covering your face.

2. If the tips of the pinkies (or baby fingers) point away from one another, the pinkies are bent (recessive trait).

Forelock• A White Forelock (dominant trait) is a patch of white hair,

usually located at the hairline just above the forehead. The photo to the left clearly shows an exaggerated white forelock.

• No White Forelock is the recessive trait.

Earlobes• Free earlobes (dominant trait) hang below the point of

attachment to the head. • Attached ear lobes (recessive trait) are attached directly to

the side of the head.

DimplesDimples: Dimples (dominant trait) vs. No dimples (recessive trait) Dimples are natural dents in the face to the right or left of the mouth. If a person has only one dimple, they should be counted as having dimples.Cleft chin is dominant over no cleft.

Hairline

• Widow's Peak (below) is dominant over no widow's peak hairline.

Widow's Peak (below) is dominant over no widow's peak hairline.

Tongue rolling

• Tongue-Rolling: Rolling up edges (dominant trait) vs not rolling (recessive)

Analysis

• Are dominant or recessive trait more prevalent in the classroom?

• Explain how this can occur