chapter 14 : Human Inheritance

. An X-linked recessive gene (c) produces red-green colorblindness. A woman with normal vision whose father was colorblind marries a colorblind man. (a) What are the possible genotypes for the mother of the colorblind man? (b) What are the possible genotypes for the father of the colorblind man? (c) What are the chances that the first son will be colorblind? (d) What are the chances that the first daughter will be colorblind?

(a) Cc or cc (b) CY or cY (c) 1/2 (d) 1/2

In cats an X-linked pair of alleles, B and b, controls color of fur. B produces black, b produces yellow, and Bb produces tortoiseshell. (a) A yellow cat had a litter of two tortoiseshell kittens and one yellow kitten. What is the sex of the yellow kitten? (b) A tortoiseshell cat brings home a litter of black, yellow, and tortoiseshell kittens. The color of which sex would tell you the color of the tomcat that produced them? (c) A yellow male cat is crossed with a tortoiseshell female cat. If the female cat has all male kittens in her litter of four, what color(s) would they be? (d) A tortoiseshell cat brings home her litter of black, yellow, and tortoiseshell kittens. By what method could you possibly decide whether the male parent was the black tomcat next door?

(a) male (b) female (c) 1/2 yellow, 1/2 black (d) black female kitten

1. D 2. A 3. B 4. C

1. ____ Which letter corresponds to an addition? 2. ____ Which letter corresponds to a deletion? 3. ____ Which letter corresponds to an inversion? 4. ____ Which letter corresponds to a translocation?

Red-green colorblindness is an X-linked recessive trait in humans. A colorblind woman and a man with normal vision have a son. What is the probability that the son is colorblind?

100 percent

A male and female both have the autosomal dominant allele for Huntington's disease. What is the likelihood that their offspring would have Huntington's disease?

100%

Down syndrome involves trisomy of this chromosome:

21

How many chromosomes are present in a gamete (sperm or egg)?

23

Following nondisjunction of one chromosome in the formation of secondary spermatocytes and their subsequent fertilization of normal ova, some of the zygotes will have 47 chromosomes and others will have _____?

45

Following a gene duplication event involving only five loci, how many chromosomes will a human have?

46

How many chromosomes are present in the somatic cells of a child born with Down syndrome (trisomy 21)?

47

How many chromosomes does each somatic cell have in a human male who has two X chromosomes?

47

A man and woman undergo genetic testing before conceiving a child. They discover that only the male is a carrier for Tay-Sachs disease, which is an autosomal recessive trait. What are the chances that their offspring will be a carrier for Tay Sachs disease as well?

50%

A man and woman undergo genetic testing before conceiving a child. They discover that they are both carriers for Tay-Sachs disease, which is an autosomal recessive trait. What are the chances that their offspring will be a carrier for Tay Sachs disease as well?

50%

As in the case for all autosomal dominant disorders, if one parent has the disorder acondroplasia but the other parent does not, what are the chances of their offspring having acondroplasia?

50%

The Philadelphia chromosome is an example of a chromosomal translocation, occurring between chromosomes 9 and 22. This translocation is associated with a form of cancer called chronic myelogenous leukemia (CML). How could a chromosomal translocation such as this lead to cancer?

A chromosomal translocation can be detrimental because it combines gene regions that are not normally associated with each other. Some translocations may combine two genes, making a new protein, which may (or may not be harmful). Some translocations can move a proto-oncogene to a region that is vigorously transcribed, with the result being uncontrolled cell divisions that are characteristic of cancer.

All chromosome pairs are similar in size and gene content except the sex chromosomes. Why are the X and Y sex chromosomes different in size and gene content?

About 350 million years ago, a gene on one of the chromosomes mutated to produce the SRY gene, which is now the determinant of the male sex. The mutation interfered with crossing over during meiosis. The reduced frequency of crossing over allowed the chromosomes to diverge around the changed region as mutations began to accumulate separately in the two chromosomes. Over evolutionary time, the chromosomes became so different that they no longer crossed over at all in the changed region, so they diverged even more.

autosomal recessive inheritance

Albinism

Hemophilia is an X-linked recessive gene. A normal woman whose father had hemophilia marries a normal man. What are the chances of hemophilia in their children?

All females would be normal, but half of them would be carriers; half of the males would have hemophilia, the other half would be normal.

Most people are not aware of chromosomal changes that they may possess. For example, why is it that some may discover they have a chromosomal inversion once they are faced with fertility problems?

An inversion may not affect a carrier's health if it does not interrupt a gene or gene control region because the individual's cells still contain their full complement of genetic material. However, fertility may be affected because a chromosome with an inversion does not pair properly with its homologous partner during meiosis. Crossovers between these mispaired chromosomes can produce other chromosome abnormalities that reduce the viability of forthcoming embryos. People with fertility issues may have their karyotype checked, which is when a chromosome inversion may be discovered.

Autosomal dominant. Both males and females are affected at similar levels which rules out X-linked inheritance and individuals are affected in each generation which is unlikely with an autosomal recessive pattern of inheritance. Autosomal dominant patterns of inheritance show affected individuals in each generation and males and females are affected evenly.

Does the phenotype indicated by black circles and squares in the pedigree below show an inheritance pattern that is autosomal dominant, autosomal recessive, or X-linked?

This disorder is also known as trisomy 21.

Down syndrome

nondisjunction of the twenty-first chromosomal pair

Down syndrome

Is this statement true or false? A son can inherit an X-linked recessive allele from his father.

False

autosomal dominant inheritance

Huntington's disease

People with this disorder age too rapidly to reproduce.

Hutchinson-Gilford progeria

Why are extra X chromosomes in females not considered a problem?

In females, all but one X chromosome is inactivated early in development (Barr bodies) so no matter how many X chromosomes the female has, only one is active in each cell.

Describe the benefits of darker skin color in areas of intense sunlight and lighter skin color in areas with less sunlight.

In hot, sunny areas, melanin has protective effects. It prevents UV radiation from breaking down folate, a vitamin essential for normal sperm formation and embryonic development. In colder areas, with less sun, UVradiation stimulates skin cells to make a molecule the body converts to vitamin D. Where sunlight exposure is minimal, UV radiation is less of a risk than vitamin D deficiency, which has serious health consequences for developing fetuses and children.

Affected males tend to be overweight, tall, and within a normal range of intelligence.

Klinefelter syndrome

If a father and a son are both colorblind and the mother has normal vision, is it likely that the son inherited colorblindness from his father?

No, males inherit all sex-linked traits from the mother.

Why is it that some autosomal dominant disorders are passed on to offspring regularly (e.g., Huntington's disease), while other autosomal dominant disorders are not (e.g., Hutchinson-Gilford progeria)?

The main difference between the two examples is that Huntington's disease is a late-onset disease, with symptoms normally showing up in individuals in their thirties, while symptoms of progeria are evident early on in life (ex. two years of age). Therefore, people who have the autosomal dominant allele for Huntington's disease may have children before they discover their disorder whereas patients with progeria often die in their early teens, before child-bearing age.

E

The pedigree above shows the inheritance of an autosomal dominant disorder. Which letter on the pedigree indicates an affected male?

Marfan syndrome is a genetic disorder that affects connective tissue within the body. People are usually tall with long limbs but there are serious symptoms as well, including cardiac problems. The allele responsible for Marfan syndrome is inherited in an autosomal dominant pattern. What is the chance that any child will have Marfan syndrome if one parent does not carry the allele and the other is heterozygous?

There is a 50% chance that their child will have Marfan sydrome.

This disorder is due to a sex chromosome abnormality probably caused by nondisjunction of sex chromosomes at meiosis.

Turner syndrome

nondisjunction of the sex chromosomes

Turner syndrome

0%

What is the chance of affected offspring from the cross "B" X "C"?

50%

What is the chance of affected offspring from the cross "B" x "D"?

25%

What is the chance of normal offspring from the cross "D" x "E"?

Aa

What is the genotype of "B"?

Aa

What is the genotype of "D"?

. The sex chromosome composition of a person with Turner syndrome is

XO

The sex chromosome composition of a person with Klinefelter syndrome is

XXY

Males who tend to be taller than average and show mild mental retardation may have the ____ chromosome complement

XYY

In humans an X-linked disorder called coloboma iridia (a fissure in the iris) is a recessive trait. A normal couple has an afflicted daughter. The husband sues the wife for divorce on the grounds of infidelity. Would you find in his favor?

Yes, the daughter would have to inherit the recessive trait from both parents.

A small region of a protein from three species is sequenced and found to be as follows: Species X is alanine, glycine, glycine, threonine, alanine. Species Y is alanine, glycine, threonine, alanine. Species Z is alanine, valine, glycine, threonine, alanine. The difference in the amino acid sequence of species Y is most likely due to ____.

a deletion

People homozygous for the HbS allele develop sickle cell anemia. Heterozygotes have few symptoms. A couple who are both heterozygous for the HbS allele plan to have children. For each of the pregnancies, state the probability that they will have a child who is: a. homozygous for the HbS allele b. homozygous for the normal allele c. heterozygous (having the normal and the HbS allele)

a. 25% b. 25% c. 50%

Four of the five answers listed below are caused by recessive genes. Select the exception. a. Huntington disorder b. phenylketonuria c. color blindness d. hemophilia e. albinism

a. Huntington disorder

Human females are XX and males are XY. a. With respect to X-linked alleles, how many different types of gametes can a male produce? b. If a female is homozygous for an X-linked allele, how many types of gametes can she produce with respect to that allele? c. If a female is heterozygous for an X-linked allele, how many types of gametes can she produce with respect to that allele?

a. With respect to X-linked alleles, a male can produce only one type of gamete. Half of the gametes he produces carry an X chromosome and half carry a Y chromosome. All the gametes that carry the X chromosome will have the X-linked allele. b. A female homozygous for an X-linked allele produces only one kind of gamete. c. Fifty percent of the gametes of a female who is heterozygous for an X-linked allele carry one of the two alleles at that locus; the other fifty percent carry its partner allele for that locus.

When a baby is born with six fingers, the doctor may explain to the parents that this trait is a genetic ____, or a rare version of a trait.

abnormality

A small region of a protein from three species is sequenced and found to be as follows: Species X is alanine, valine, threonine, alanine. Species Y is alanine, glycine, threonine, alanine. Species Z is alanine, valine, glycine, threonine, alanine. The differences in the amino acid sequence of species Z is most likely due to ____.

an addition

when too many or too few copies of a particular chromosome are inherited

anuploid

Four of the five answers listed below are conditions caused by chromosomal nondisjunction. Select the exception. a. Down syndrome b. Hemophilia c. Turner syndrome d. Klinefelter syndrome e. Trisomy 21

b. Hemophilia

If a daughter has an X-linked recessive disorder, such as color-blindness, she inherited the trait from

both parents

Amniocentesis is

c. used in prenatal diagnosis to detect chromosomal mutations and disorders in embryos.

Which membrane that helps form the placenta and can yield cells for early prenatal diagnosis?

chorion

In a pedigree chart, a female who does not demonstrate the trait being studied is represented by a

clear circle

Four of the five answers listed below are related conditions in which abnormal numbers of chromosomes are present. Select the exception. a. Turner syndrome b. Down syndrome c. nondisjunction d. Huntington's disease e. trisomy

d. Huntington's disease

In a pedigree chart, a male showing the specific trait being studied is indicated by a

darkened square

A chromosome's gene sequence that was ABCDEFG before damage and ABCFG after is an example of

deletion.

The inheritance of a certain allele of a gene that leads to medical problems indicates a genetic ____.

disorder

A chromosome's gene sequence that was ABCDEFG before modification and ABCDCDEFG afterward is an example of

duplication

Which genetic disorders is not a polygenic trait? a. asthma b. obesity c. multiple sclerosis d. schizophrenia e. cystic fibrosis

e. cystic fibrosis

Red-green colorblindness is an X-linked recessive trait. Two normal-visioned parents have a colorblind son. Indicate the genotype and phenotype of each parent and the son.

father CY, mother Cc, son cY

This disorder is the result of an X-linked recessive trait.

hemophilia

Which disorder is more common in males?

hemophilia

X-linked recessive inheritance

hemophilia

Which X-linked recessive trait affects an individual's blood clotting ability?

hemophilia A

An X-linked recessive carrier is a

heterozygous female.

A chromosome that has been broken and rejoined in a reversal sequence has undergone

inversion

A chromosome's gene sequence that was ABCDEFG before damage and ABCGFED after is an example of

inversion

Alleles associated with Tay-Sach's disease have mutations that cause a(n)

malfunction of a lysosomal enzyme.

In humans, skin cell organelles called _____ produce pigment

melanosomes

(2n -1); a gamete deprived of a chromosome

monosomy

Sometimes homologous chromosomes fail to separate during meiosis, resulting in one daughter cell with an extra chromosome and one daughter cell missing a chromosome. This failure of chromosome separation is called:

nondisjunction

A human X-linked recessive gene may be

passed to daughters from their fathers.

. When studying a single-gene inherited disorder such as Huntington's disease, researchers may construct a chart to look for family connections. What type of chart is this?

pedigree

Symptoms of phenylketonuria (PKU) may be minimized or suppressed by a diet low in

phenylalanine.

Which trait listed below would be considered a genetic abnormality, rather than a genetic disorder?

polydactyly

a species with three or more complete sets of chromosomes

polyploid

Suppose a hemophilic male (X-linked recessive allele) and a female carrier for the hemophilic trait have a nonhemophilic daughter with Turner syndrome. Nondisjunction could have occurred in

the father only

Amniocentesis involves sampling

the fetal cells floating in the amniotic fluid

Which DNA segments can spontaneously move to another chromosome region?

transposable elements

(2n +1); three chromosomes of the same kind are present in a set of chromosomes

trisomy

The condition occurring when an organism has a full set of chromosomes (2n) + 1 chromosome is known as

trisomy.