13. Mendelian Genetics

Sex-Linked Inheritance

Consider the evolution of protostomes and deuterostomes (this chapter): (1) Compare adaptations in protostomes and deuterostomes for living on land. (2) Recall that changes in the expression patterns of tool-kit genes in arthropods such as insects enabled a dramatic diversification of their segmented appendages and bodies. Design an experiment to test whether this concept applies to vertebrates.

Y-Linked Traits in a Pedigree

X Linked Inheritance Explained

 Bony ray males can pass on bony rays only to their male offspring. 

 Females with either bony or soft rays can reproduce, but males with soft rays cannot transfer sperm to the female. 

 No. An ability or inability to reproduce is not indicative of X linkage. 

 Females with bony rays cannot pass bony rays to female offspring even if they are mating with a bony ray male. 

 No. Females can pass X-linked traits to their offspring. 

 All of the listed responses are correct. 

 No. Only one of the other choices is correct. 

 Matings of soft ray males and bony ray females give different results from the matings of bony ray males and soft ray females. 

This pattern should be the same as that seen in X-linked human traits such as hemophilia.

In a certain fish, fin rays (supporting structures for the fins) can be either bony or soft in adult fish. Sex linkage in a fish is similar to that in humans. What evidence would most strongly support the idea that the ray locus is on the X chromosome?

Pseudohypertrophic muscular dystrophy is an inherited disorder that causes gradual deterioration of the muscles. It is seen almost exclusively in boys born to apparently unaffected parents and usually results in death in the early teens. Is this disorder caused by a dominant or a recessive allele? Is its inheritance sex-linked or autosomal? How do you know? Explain why this disorder is almost never seen in girls.

Punnett Squares and Sex-Linked Traits

The gene involved is located on the Y chromosome.

The gene involved is located on the X chromosome.

The gene involved is located on an autosome, but only in males.

Other male-specific factors influence eye color in flies.

When Thomas Hunt Morgan crossed red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies. Remarkably, all the white-eyed flies were male. What was the explanation for this result?

The size and shape of the vertebrate skull can reveal a great deal about an animal’s lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don’t. Where did ear bones come from?

The illustration of the opossum skull shows that the ear bones are completely separated from the jawbone (as they are in all mammals). Pose a hypothesis to explain why this separation could be an adaptation that contributed to the radiation of mammals into diverse niches, including a nocturnal lifestyle.

X-Linked Recessive Pedigrees

Draw a Punnett square. See for more information on hemophilia.

Hemophilia is an X-linked disorder. The daughter of a father with hemophilia and a carrier mother has a __________ probability of having hemophilia.

 Only X-linked genes are technically sex-linked because Y-linked genes affect characteristics unrelated to sex. 

 No. Both are considered sex-linked because they are genes located on the sex chromosomes. Also, most Y-linked genes help determine sex. 

 Sons and daughters can inherit alleles of X-linked genes from fathers proportionally, but only sons inherit alleles of Y-linked genes from their fathers. 

 No. This answer is partially correct. Recall that only daughters inherit an X chromosome from their fathers.  

 Genes on the X chromosome are expressed only in females, and genes on the Y chromosome are expressed only in males. 

 No. Y-linked genes are expressed only in males because females don't carry Y chromosomes. Most X-linked genes, however, determine traits unrelated to sex and are expressed in both males and females. 

 Though most X-linked genes determine sex, Y-linked genes control for characters unrelated to sex. 

 No. Most Y-linked genes help determine sex, whereas the many X-linked genes code for characters unrelated to sex. 

 Sons and daughters have equal probabilities in inheriting a recessive allele of an X-linked gene from their mother, but only sons can inherit rare Y-linked genetic disorders from their father.  

Consider how the inheritance of sex chromosomes (and their alleles) can affect sons and daughters differently.

X-linked genes differ from Y-linked genes in which of the following ways?

 No. A son does not inherit an X from his father. 

 No. A male cannot trace his X chromosome to his paternal grandmother. 

 No. However, a male can trace his Y chromosome to his paternal grandfather. 

 None of the listed responses is correct. 

 No. One of the other choices is correct. 

Consider the chromosomal complement of a human male.

For an X-linked trait, it is the contribution of __________ that determines whether a son will display the trait.

 No. The father only has a single, normal allele. 

Draw a Punnett square. See for more information on color blindness.

A woman is a carrier for red-green color blindness, a sex-linked trait. Her husband is normal (not color-blind) for this trait. What are the chances that their newborn daughter will be red-green color-blind?

X-Linked Recessive Disorder: Hemophilia Inheritance

Sex-Linked Inheritance Example 1

The following pedigree is for the X-linked-recessive trait for color blindness. Using XN for the normal allele and Xn for the color blindness allele, fill in the top half of the boxes/circles with the genotype. Also, fill in the bottom half of the boxes/circles with the phenotype (Normal vision or color blind). If it is impossible to know for certain a specific allele in the genotype, then place a “?” to represent the allele that is in question.<img src="https://cdn.clutchprep.com/problem_images/147228-970bde934d426e51.png" alt="" />

The size and shape of the vertebrate skull can reveal a great deal about an animal’s lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don’t. Where did ear bones come from?

How is the opossum related to you? Select True or False for each statement.
T/F An opossum is an animal, but I am a human.
T/F An opossum is a mammal, but I am a human.
T/F An opossum is a marsupial, but I am a placental mammal.
T/F The opossum and I are both tetrapods.

A woman is a carrier of the X-linked recessive color blindness gene. She has children with a man with normal color vision. Which of the following is true of their offspring?
a. All the males will be color blind;
b. All the females will be carriers;
c. Half the females will be color blind;
d. Half the males will be color blind.

Draw a Punnett square. See for more information on sex-linked traits.

In werewolves (hypothetically), the pointy ear allele (P) is dominant over the round ear allele (p). The gene for ear shape is on the X chromosome. (Sex determination in werewolves is the same as for "other" humans.) A certain female werewolf has pointy ears even though her father had round ears. What percentage of her sons will have round ears if she mates with a werewolf with round ears?

 Either her mother was homozygous dominant or her father had hemophilia. 

 No. A homozygous dominant mother could not be responsible for a carrier daughter. 

 Both her mother and her father had hemophilia. 

 No. In this case, the woman would have had hemophilia. 

 Either her mother was a carrier or her father had a dominant allele. 

 No. A father with a dominant allele could not be responsible for a carrier daughter. 

 Either her mother was a carrier or her father had hemophilia. 

Draw a pedigree. For more information on hemophilia, see .

Consider a woman who is a carrier of a recessive X-linked allele for hemophilia. Which of the following possibilities could explain the presence of the hemophilia allele in her genotype?

In the parental cross, a white-eyed male was crossed with a red-eyed female. The F1 progeny all had red eyes.

 No. This result would be expected with strictly dominant/recessive alleles. 

The F1 flies were crossed with each other, and the F2 progeny included red-eyed and white-eyed flies in a 3:1 ratio.

 The mutant allele (white eyes) was recessive. 

 No. This result was not unexpected. Either allele could be recessive according to Mendelian genetics. 

 No. All but one of the answers follow normal Mendelian laws. 

Among the F2 progeny, only males had white eyes. All of the females had red eyes.

The gene is on the X chromosome.

Which of the following results of Thomas Hunt Morgan's experiments with white-eyed mutant flies was unexpected in light of Mendelian genetics?

Wild type fruit flies have red eyes. A white-eyed female fly is crossed with a red-eyed male fly. All of the females from the cross are red-eyed and all of the males, white-eyed. What type of inheritance pattern is this?

Inheritance of X-Linked Genes

 We have too little information to tell. 

 No. There is sufficient information to answer the question. 

 There is a 50% probability that he has normal vision. 

 No. A normal male cannot father a red-green color-blind daughter. 

 No. Because this color vision gene is on the X chromosome, it doesn't matter how many Y chromosomes the father has, so long as he has a single X chromosome. 

Red-green color blindness is an X-linked recessive trait.

A woman is red-green color-blind. What can we conclude, if anything, about her father?

A man with hemophilia (a recessive, sex-linked condition) has a daughter without the condition. She marries a man who does not have hemophilia. What is the probability that their daughter will have hemophilia? Their son? If they have four sons, what is the probability that all will be affected?

 No. The correct answer is 0% because a son cannot receive an X chromosome from his father. 

Consider how "maleness" is determined.

What is the probability that a human male will inherit an X-linked recessive allele from his father?

 All of their sons will show the recessive trait. 

 None of their sons will show the recessive trait. 

 All of their daughters will be heterozygous. 

 No. Their mother is heterozygous. Any daughters would have a 50% chance of being heterozygous. 

 Half of their sons will show the recessive trait. 

If a woman is heterozygous for a recessive sex-linked allele and her husband has the dominant allele, which one of the following is true about the probabilities for their children?

The size and shape of the vertebrate skull can reveal a great deal about an animal’s lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don’t. Where did ear bones come from?

Analyze the morphological data shown here and write a hypothesis to explain the origin of mammalian ear bones. (The cynodont shown is one of many extinct synapsid amniotes that lived early in the lineage that gave rise to mammals.)


X-Linked Inheritance

Sex Linked Traits: Baldness and Hemophilia

 Sex-linked traits are never seen in girls. 

 No. Females can express an X-linked recessive allele if they carry the allele on both X chromosomes. 

 The allele is carried on the Y chromosome. 

 No. An X-linked gene causes Duchenne muscular dystrophy. 

 Nondisjunction occurs in males but not in females. 

 No. Nondisjunction occurs in both males and females. 

 A sex-linked allele cannot be passed from mother to daughter. 

 No. A daughter receives an X chromosome from her mother. 

 To express an X-linked recessive allele, a female must have two copies of the allele, one of which is contributed by the father. 

Consider why males are more likely to exhibit a sex-linked recessive trait.

Duchenne muscular dystrophy is caused by a sex-linked recessive allele. Its victims are almost invariably boys, who usually die before the age of 20. Why is this disorder almost never seen in girls?

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