Genetics Problem Set: Solutions

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Problem #1: Basic Monohybrid Cross

The ability to roll the tongue is dominant over the inability to do so in humans.

(A) If two heterozygous tongue-rollers have children, what genotypes could their children have?

Using T=tongue-rolling and t=non-tongue-rolling, the genotypes and ratios are - 1 TT : 2 Tt : 1 tt. See Punnett square below.

	T	t
T	TT Tongue-roller	Tt Tongue-roller
t	Tt Tongue-roller	tt Non-tongue-roller

(B) If a non-tongue-roller has children with a homozygous tongue-roller, what will their children's genotypes be?

All the children will be tongue-rollers. See Punnett square below.

	T	T
t	Tt Tongue-roller	Tt Tongue-roller
t	Tt Tongue-roller	Tt Tongue-roller

Problem #2

In an alien species, eye colors can be black, white, or gray. A white-eyed mother and a black-eyed father have all gray-eyed children.

(A) What genetic pattern is most likely demonstrated here?

The genetic pattern most likely demonstrated in this example is that of codominance, or incomplete dominance.

(B) A gray-eyed couple has two babies: one white-eyed and one black-eyed. Does this confirm or deny your answer to part A? If necessary, revise your hypothesis.

This new evidence confirms the hypothesis of incomplete dominance. See Punnett square below (W=black eyes, w=white eyes).

	W	w
W	WW Black eyes	Ww Gray eyes
w	Ww Gray eyes	ww White eyes

This is not possible. See Punnett square below (W=black eyes, w=white eyes).

	W	w
W	WW Black eyes	Ww Gray eyes
W	WW Black eyes	Ww Gray eyes

Problem #3

A mother has blood type A and genotype AO. A father has blood type B and genotype BO. What blood types are possible for the offspring?

The genotypes AO (type A blood), BO, (type B blood), AB (type AB blood), and OO (type O blood) are all possible. See Punnett square below.

	A	O
B	AB Type AB	BO Type B
O	AO Type A	OO Type O

Problem #4

Pattern baldness is sex-linked, but cannot be expressed normally in females.

(A) If a carrier female marries a pattern-bald man, what are the possible genotypes of their children?

Using X^B as bald and X as normal, the genotypes possible are: X^BX^B, XX^B, X^BY, XY.

	X^B	X
X^B	X^BX^B Affected female (not expressed)	XX^B Carrier female
Y	X^BY Affected male	XY Normal male

(B) If female is homozygous for pattern baldness (but does not express the trait due to hormones), and she marries a normal man, what percentage of her sons will be pattern-bald?

Using X^B as bald and X as normal, the genotypes possible are: X^BX, XX^B, X^BY, X^BY. Therefore her sons will all be pattern-bald.

	X^B	X^B
X	X^BX Carrier female	X^BX Carrier female
Y	X^BY Affected male	X^BY Affected male

Problem #5

In pea plants, tallness is dominant over shortness and smooth seeds are dominant over wrinkled seeds. If two heterozygous tall plants which are heterozygous for smooth seeds are crossed, what are the genotypes and phenotypes of the offspring?

The phenotypic ratios are as follows: 9 tall smooth : 3 tall wrinkled : 3 short smooth : 3 short wrinkled. See Punnett square below (T=tall, t=short, S=smooth, and s=wrinkled).

	TS	Ts	tS	ts
TS	TTSS Tall smooth	TTSs Tall smooth	TtSS Tall smooth	TtSs Tall smooth
Ts	TTSs Tall smooth	TTss Tall wrinkled	TtSs Tall smooth	Ttss Tall wrinkled
tS	TtSS Tall smooth	TtSs Tall smooth	ttSS Short smooth	ttSs Short smooth
ts	TtSs Tall smooth	Ttss Tall wrinkled	ttSs Short smooth	ttss Short wrinkled

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