Chapter 23

In keeping with Mendel's law of independent assortment, an individual heterozygous for two traits can form _____ types of gametes.

4

pedigree

Chart of genetic relationship of family individuals across generations.

Marfan syndrome and Huntington disease are both _______ disorders.

autosomal dominant

An individual has _____ allele(s) for each trait

two

in keeping with Mendel's law of segregation, each heterozygous individual can form _____ type(s) of gametes.

two

Homozygous dominant is indicated by

two capital letters

Homozygous recessive is indicated by

two lowercase letters

Can environmental factors affect the expression of certain traits?

yes

Tay-Sachs disease, cystic fibrosis, phenylketonuria, and sickle cell disease are all ________ disorders.

autosomal recessive

gametes have ____ allele(s) for each trait

one

It may be possible to decide if an inherited condition is due to an autosomal dominant or an autosomal recessive allele by studying a

pedigree

The results (Punnett square) may be expressed as a

probable phenotypic ratio

When a heterozygote reproduces with a heterozygote, the phenotypic ratio is

3:1 that is, each child has a 75% chance of having the dominant phenotype and a 25% chance of having the recessive phenotype

the phenotypic ratio for a dihybrid cross is

9:3:3:1 in other words, 9/16 of the offspring have the two dominant traits,3/16 have one of the dominant traits with one of the recessive traits,3/16 have the other dominant trait with the other recessive trait, and1/16 have both recessive traits.

dominant alleles

Allele that exerts its phenotypic effect in the heterozygote; it masks the expression of the recessive allele.

recessive alleles

Allele that exerts its phenotypic effect only in the homozygote; its expression is masked by a dominant allele.

alleles

Alternative form of a gene; alleles occur at the same locus on homologous chromosomes

Marfan syndrome

Autosomal dominant genetic disorder of the connective tissue, specifically the fibrillin protein.

huntington disease

Autosomal dominant genetic disorder that affects the nervous system; results in a progressive loss of neurons in the brain.

phenylketonuria (PKU)

Autosomal recessive genetic disorder that causes a lack of the enzyme that metabolizes phenylalanine; the accumulation of phenylalanine causes problems with nervous system development and function.

sickle cell disease

Autosomal recessive genetic disorder that causes a malformation of hemoglobin molecules, causing red blood cells to form a sickle shape; also sometimes called sickle cell anemia due to the symptoms of the disease.

Tay-Sachs disease

Autosomal recessive genetic disorder that results in a deficiency in the enzyme hexosaminidase A; causes an accumulation of glycolipids in the lysosomes, resulting in a progressive loss of psychomotor functions.

testcross

Cross between an individual with a dominant phenotype and an individual with a recessive phenotype to determine whether the dominant individual is homozygous or heterozygous.

monohybrid cross

Cross between parents that differ in only one trait.

dihybrid cross

Cross between parents that differ in two traits

genotype

Genes of an organism for a particular trait or traits; often designated by letters—for example, BB or Aa.

cystic fibrosis (CF)

Genetic disease caused by a defect in the CFTR gene, which is responsible for the formation of a transmembrane chloride ion transporter; causes the mucus of the body to be viscous.

familial hypercholesterolemia

Genetic disorder that causes an accumulation of cholesterol in the blood due to defects in the LDL-receptors on the cell surface.

incomplete dominance

Inheritance pattern in which an offspring has an intermediate phenotype, as when a red-flowered plant and a white-flowered plant produce pink-flowered offspring. ex. Curly hair

codominance

Inheritance pattern in which both alleles of a gene are equally expressed in a heterozygote.

multiple alleles

Inheritance pattern in which there are more than two alleles for a particular trait; each individual has only two of all possible alleles. ex. ABO blood types

law of independent assortment

Mendelian principle that explains how combinations of traits appear in gametes

law of segregation

Mendelian principle that explains how, in a diploid organism, alleles separate during the formation of the gametes.

homozygous

Possessing two identical alleles for a particular trait.

heterozygous

Possessing unlike alleles for a particular trait.

polygenic inheritance

Pattern of inheritance in which a trait is controlled by several allelic pairs.

locus

Physical location of a trait (or gene) on a chromosome.

phenotype

Visible expression of a genotype—e.g., brown eyes or attached earlobes.

punnett squares

Visual representation developed by Reginald Punnett that is used to calculate the expected results of simple genetic crosses.

According to the law of independent assortment, a. all possible combinations of factors can occur in the gametes. b. only the parental combinations of gametes can occur in the gametes. c. only the nonparental combinations of gametes can occur in the gametes.

a

The genotype of an individual with the dominant phenotype can be determined best by reproduction with a. a homozygous recessive individual. b. an individual with the dominant phenotype. c. a heterozygote. d. None of these are correct.

a

Heterozygous is indicated by

a capital letter and a lowercase letter.

Genes carry instructions for

a trait

(Punnett square) When performing an actual cross, it is assumed that

all possible types of sperm fertilize all possible types of oocytes.

Alternative forms of a gene (producing differences in the trait) are called

alleles

In a dihybrid cross of AaBb × AaBb, what is the probability that offspring will have the dominant phenotype for trait A but the recessive phenotype for trait a. 1/16 b. 3/16 c. 7/16 d. 9/16 e. None of these are correct.

b

Using the product rule, determine the probability that an Aa individual will be produced from an Aa × Aa cross. a. 25% b. 50% c. 75% d. 100% e. 0%

b

Which of the following does not describe the pedigree analysis of an autosomal dominant disorder? a. heterozygotes are affected b. two affected parents will always have affected children c. affected children usually have an affected parent d. two unaffected parents will not have affected children

b

Which of the following does not have an autosomal recessive pattern of inheritance? a. cystic fibrosis b. Huntington disease c. phenylketonuria d. sickle cell disease e. None of these are correct.

b

A trait in which the heterozygotes have a phenotype that is intermediate between the dominant and recessive phenotypes is an indication of a. codominance. b. pleiotropy. c. a sex-linked trait. d. incomplete dominance. e. None of these are correct.

d

If a child has type O blood and the mother is type A, then which of the following might be the blood type of the child's father? a. A only b. A or O c. B only d. A, B, or O e. O only

d

In the analysis of a genetic disorder, you notice that the phenotypic distribution follows a bell-shaped curve, and that multiple genes are probably involved. This suggests what pattern of inheritance? a. autosomal dominant b. autosomal recessive c. incomplete dominance d. polygenic inheritance e. None of these are correct.

d

The law of segregation states all of the following except a. factors separate during formation of the gametes. b. each individual has two factors for each trait. c. gametes contain a single factor for each trait. d. factors assort independently of each other by meiosis.

d

Which of the following does not have an environmental factor that affects the distribution of the phenotype? a. phenylketonuria b. skin color c. height d. Marfan syndrome

d

In Siamese cats and Himalayan rabbits, the effect of temperature is apparent in the

distribution of fur color.

If two parents with short fingers (dominant) have a child with long fingers, what is the chance their next child will have long fingers? a. There is no chance. b. 1/16 c. 1/2 d. 3/16 e. 1/4

e

In polygenic inheritance, such as height and skin color in humans, several genes each contribute to the overall phenotype in ______ degrees

equal, small

There are many exceptions to Mendel's laws, including

incomplete dominance codominance and multiple alleles

Familial hypercholesterolemia is a(n) _______ disorder.

incompletely dominant autosomal

It is customary to use _______ to represent the genotypes of individuals.

letters

Humans can _______ to lessen the damaging effects of genetic defects.

manipulate the environment

For a trait, dominant alleles mask

the expression of recessive alleles.

A testcross is sometimes used to determine

the genotype of an individual with a dominant phenotype.

Punnett squares are often used to indicate

the potential offspring of a genetic cross.