Chapter 10

Question 1

What is heredity?

Answer 1

The tendency for traits to be passed from parent to offspring.

Question 2

What are traits?

Answer 2

The expressions of a character or heritable feature in an organism.

Question 3

What is the P generation in Mendel’s experiments?

Answer 3

The true-breeding parental plants used in the original cross.

Question 4

What is the F1 generation?

Answer 4

The first filial generation — the offspring produced from crossing two P generation plants.

Question 5

What is the F2 generation?

Answer 5

The second filial generation — the offspring produced when F1 plants self-fertilize.

Question 6

What did Mendel observe in the F1 generation?

Answer 6

All F1 plants expressed only one of the two traits — the dominant trait; the other trait disappeared.

Question 7

What did Mendel observe in the F2 generation?

Answer 7

The recessive trait reappeared — approximately 3/4 of plants showed the dominant trait and 1/4 showed the recessive trait, a 3:1 ratio.

Question 8

What did Mendel find when F2 plants self-fertilized to produce F3 offspring?

Answer 8

The 3:1 ratio was actually a disguised 1:2:1 ratio — 1 true-breeding dominant : 2 not-true-breeding dominant : 1 true-breeding recessive.

Question 9

What is a dominant trait?

Answer 9

The trait expressed in the F1 generation when two contrasting traits are crossed — it masks the recessive trait in heterozygotes.

Question 10

What is a recessive trait?

Answer 10

The trait that disappears in the F1 generation but reappears in 1/4 of F2 offspring — only expressed when two copies of the recessive allele are present.

Question 11

What are genes?

Answer 11

The factors Mendel called merkmal — units of information that determine traits and are passed from parent to offspring.

Question 12

What are alleles?

Answer 12

Alternative forms of a gene that lead to alternative traits.

Question 13

What does homozygous mean?

Answer 13

An individual that carries two identical alleles for a gene (e.g., PP or pp).

Question 14

What does heterozygous mean?

Answer 14

An individual that carries two different alleles for a gene (e.g., Pp).

Question 15

What is genotype?

Answer 15

The specific alleles an individual carries for a gene.

Question 16

What is phenotype?

Answer 16

The physical appearance or expression of an individual’s genotype.

Question 17

How are dominant and recessive alleles typically written?

Answer 17

Dominant alleles are written in uppercase letters (e.g., P); recessive alleles in lowercase (e.g., p).

Question 18

What is a Punnett square?

Answer 18

A diagram used to predict the probable genotypes and phenotypes of offspring from a cross by listing parental gametes along the sides.

Question 19

What is the predicted genotype ratio from a cross of two heterozygotes (Pp × Pp)?

Answer 19

1 PP : 2 Pp : 1 pp — or 25% homozygous dominant, 50% heterozygous, 25% homozygous recessive.

Question 20

What is the predicted phenotype ratio from a cross of two heterozygotes (Pp × Pp)?

Answer 20

3 dominant : 1 recessive (3:1).

Question 21

What is a testcross?

Answer 21

Crossing an individual of unknown genotype showing the dominant phenotype with a homozygous recessive individual to determine whether the unknown is homozygous or heterozygous.

Question 22

What result indicates a test plant is homozygous dominant (PP)?

Answer 22

All offspring show the dominant phenotype.

Question 23

What result indicates a test plant is heterozygous (Pp)?

Answer 23

Half the offspring show the dominant phenotype and half show the recessive phenotype — a 1:1 ratio.

Question 24

What is Mendel’s law of segregation?

Answer 24

The two alleles of a trait separate during the formation of gametes so that half carry one allele and half carry the other.

Question 25

What is Mendel's law of independent assortment?

Answer 25

The inheritance of one trait does not influence the inheritance of another — genes on different chromosomes are inherited independently.

Question 26

What is a dihybrid cross?

Answer 26

A cross that examines the inheritance of two separate traits simultaneously.

Question 27

What phenotype ratio is expected in the F2 generation of a dihybrid cross?

Answer 27

9:3:3:1 — 9 dominant for both : 3 dominant for first only : 3 dominant for second only : 1 recessive for both.

Question 28

How do genes determine phenotype?

Answer 28

DNA encodes the amino acid sequences of proteins — proteins carry out cellular functions that determine an organism's appearance and traits.

Question 29

What is polygenic inheritance?

Answer 29

When multiple genes contribute cumulatively to a single phenotype — producing continuous variation rather than discrete categories.

Question 30

What is continuous variation?

Answer 30

A gradation of phenotypes across a range rather than two or a few distinct categories — for example human height.

Question 31

What is pleiotropy?

Answer 31

When a single gene influences more than one trait — for example the cystic fibrosis gene affects multiple organs.

Question 32

What is incomplete dominance?

Answer 32

When neither allele is fully dominant — heterozygotes express a phenotype intermediate between the two homozygous phenotypes (e.g., red × white = pink).

Question 33

What are epigenetic modifications?

Answer 33

Environmental changes that add or remove chemical groups to DNA or chromosome-packaging proteins — can affect gene expression and even be passed to future generations.

Question 34

What is codominance?

Answer 34

When two alleles are both fully expressed in a heterozygote — neither is dominant over the other (e.g., ABO blood type AB).

Question 35

What are the four ABO blood types and their genotypes?

Answer 35

Type A (IAIA or IAi), Type B (IBIB or IBi), Type AB (IAIB), Type O (ii).

Question 36

Why is blood type O called the universal donor?

Answer 36

Type O red blood cells carry no galactose or galactosamine antigens and so trigger no immune response in recipients of other blood types.

Question 37

Why is blood type AB called the universal recipient?

Answer 37

AB individuals have both antigens on their red blood cells so neither galactose nor galactosamine is foreign to them.

Question 38

What are autosomes?

Answer 38

The 22 pairs of chromosomes in humans that are perfectly matched in both males and females — not involved in sex determination.

Question 39

What are sex chromosomes?

Answer 39

The 23rd pair of chromosomes — XX in females and XY in males; the Y chromosome carries genes that determine maleness.

Question 40

What is nondisjunction?

Answer 40

The failure of chromosomes or sister chromatids to separate correctly during meiosis — resulting in gametes with too many or too few chromosomes.

Question 41

What is aneuploidy?

Answer 41

An abnormal number of chromosomes resulting from nondisjunction.

Question 42

What is Down syndrome?

Answer 42

A condition caused by trisomy 21 — an extra copy of chromosome 21 — resulting in intellectual disability and developmental delays.

Question 43

Why are older mothers at higher risk of having a child with Down syndrome?

Answer 43

All eggs a female will ever produce are present at birth — over time they can accumulate damage that leads to nondisjunction.

Question 44

What is Klinefelter syndrome?

Answer 44

A condition (XXY) resulting from an XX egg fertilized by a Y sperm — produces a sterile male with some female body characteristics.

Question 45

What is Turner syndrome?

Answer 45

A condition (XO) resulting from an O egg fertilized by an X sperm — produces a sterile female of short stature with immature sex organs.

Question 46

What is a mutation?

Answer 46

An accidental change in a gene — a random alteration that can alter or destroy the function of a protein.

Question 47

What is a genetic disorder?

Answer 47

A condition caused by a mutant allele that has become more common in a population and produces harmful effects.

Question 48

What is hemophilia?

Answer 48

A sex-linked recessive disorder in which blood clots slowly or not at all — caused by mutation in clotting factor genes on the X chromosome.

Question 49

Why do males more often express sex-linked recessive disorders like hemophilia?

Answer 49

Males have only one X chromosome — if that X carries the mutant allele there is no second copy to mask it.

Question 50

What is sickle-cell disease?

Answer 50

A recessive hereditary disorder caused by a single amino acid change in beta-hemoglobin — causes red blood cells to become sickle-shaped, obstructing blood vessels.

Question 51

Why is the sickle-cell allele common in populations from central Africa?

Answer 51

Heterozygous individuals (carriers) have increased resistance to malaria — a major selective advantage in regions where malaria is common.

Question 52

What is a pedigree?

Answer 52

A diagram showing family relationships and which individuals express a trait — used to determine whether a trait is dominant or recessive and whether it is autosomal or sex-linked.