Patterns of Inheritance

Question 1

Heredity

Answer 1

is the transmission of traits from one generation to the next.

Question 2

Genetics

Answer 2

is the scientific study of heredity.

Question 3

Gregor Mendel

Answer 3

began the field of genetics in the 1860s,
deduced the principles of genetics by breeding garden peas, and
relied upon a background of mathematics, physics, and chemistry.

Question 4

In 1866, Mendel

Answer 4

correctly argued that parents pass on to their offspring discrete “heritable factors” and
stressed that the heritable factors (today called genes), retain their individuality generation after generation.

Question 5

a character

Answer 5

A heritable feature that varies among individuals, such as flower color, is called a character.

Question 6

trait

Answer 6

Each variant for a character, such as purple or white flowers, is a trait.

Question 7

True-breeding

Answer 7

varieties result when self-fertilization produces offspring all identical to the parent.

Question 8

hybrids

Answer 8

The offspring of two different varieties are hybrids.
The cross-fertilization is a hybridization, or genetic cross.
True-breeding Parental plants are the P generation.

Question 9

monohybrid cross

Answer 9

A cross between two individuals differing in a single character is a monohybrid cross.

Question 10

Mendel developed 4 hypotheses

Answer 10

1. Alleles are alternative versions of genes that account for variations in inherited characters.
2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different.
   A homozygous genotype has identical alleles.
   A heterozygous genotype has two different alleles.
3. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.
4. The phenotype is the appearance or expression of a trait.
   The genotype is the genetic makeup of a trait.
   A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.

Question 11

A Punnett square

Answer 11

A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine.

Question 12

A locus (plural, loci)

Answer 12

the specific location of a gene along a chromosome.
For a pair of homologous chromosomes, alleles of a gene reside at the same locus.
Homozygous individuals have the same allele on both homologues.
Heterozygous individuals have a different allele on each homologue.

Question 13

A dihybrid cross

Answer 13

A dihybrid cross is a mating of parental varieties that differ in two characters.

Question 14

law of independent assortment

Answer 14

Mendel
suggested that the inheritance of one character has no effect on the inheritance of another,
suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and
called this the law of independent assortment.

Question 15

A testcross

Answer 15

A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual.
A testcross can show whether the unknown genotype includes a recessive allele.
Mendel used testcrosses to verify that he had true-breeding genotypes.
The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes.

Question 16

Wild-type traits

Answer 16

Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles.
The inheritance of human traits follows Mendel’s laws.

Question 17

A pedigree

Answer 17

shows the inheritance of a trait in a family through multiple generations,
demonstrates dominant or recessive inheritance, and
can also be used to deduce genotypes of family members.

Question 18

Inherited human disorders show either

Answer 18

recessive inheritance in which
two recessive alleles are needed to show disease,
heterozygous parents are carriers of the disease-allele, and
the probability of inheritance increases with inbreeding, mating between close relatives.
dominant inheritance in which
one dominant allele is needed to show disease and
dominant lethal alleles are usually eliminated from the population.

Question 19

Dominant human disorders include

Answer 19

achondroplasia, resulting in dwarfism, and
Huntington’s disease, a degenerative disorder of the nervous system.
The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is
recessive and
carried by about 1 in 31 Americans

Question 20

complete dominance

Answer 20

Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance.

Question 21

incomplete dominance

Answer 21

For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which
neither allele is dominant over the other and
expression of both alleles occurs.
Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation.
One example of incomplete dominance in humans is hypercholesterolemia, in which
dangerously high levels of cholesterol occur in the blood and
heterozygotes have intermediately high cholesterol levels.

Question 22

codominance

Answer 22

The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance.
In codominance,
neither allele is dominant over the other and
expression of both alleles is observed as a distinct phenotype in the heterozygous individual.
AB blood type is an example of codominance.

Question 23

Pleiotropy

Answer 23

occurs when one gene influences many characteristics.
Sickle-cell disease is a human example of pleiotropy. This disease
affects the type of hemoglobin produced and the shape of red blood cells and
causes anemia and organ damage.
Sickle-cell and nonsickle alleles are codominant.
Carriers of sickle-cell disease are resistant to malaria.

Question 24

polygenic inheritance

Answer 24

Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.
Human skin color is an example of polygenic inheritance.

Question 25

Many characters result from a combination of heredity and the environment. For example……..

Answer 25

skin color is affected by exposure to sunlight,
susceptibility to diseases, such as cancer, has hereditary and environmental components, and
identical twins show some differences.
Only genetic influences are inherited.

Question 26

The chromosome theory of inheritance states that

Answer 26

genes occupy specific loci (positions) on chromosomes and
chromosomes undergo segregation and independent assortment during meiosis.
Mendel’s laws correlate with chromosome separation in meiosis.
The law of segregation depends on separation of homologous chromosomes in anaphase I.
The law of independent assortment depends on alternative orientations of chromosomes in metaphase I.

Question 27

linked genes

Answer 27

ateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which
are located close together on the same chromosome and
tend to be inherited together.

Question 28

recombination frequency

Answer 28

Crossing over between homologous chromosomes produces new combinations of alleles in gametes.
Linked alleles can be separated by crossing over, forming recombinant gametes.
The percentage of recombinants is the recombination frequency

Question 29

Many animals have a pair of sex chromosomes

Answer 29

designated X and Y,

that determine an individual’s sex.

Question 30

In mammals,

Answer 30

males have XY sex chromosomes,
females have XX sex chromosomes,
the Y chromosome has genes for the development of testes, and
an absence of the Y allows ovaries to develop.

Question 31

In some animals, environmental temperature determines the sex.

Answer 31

For some species of reptiles, the temperature at which the eggs are incubated during a specific period of development determines whether the embryo will develop into a male or female.
Global climate change may therefore impact the sex ratio of such species.

Question 32

Sex-linked genes

Answer 32

are located on either of the sex chromosomes.
The X chromosome carries many genes unrelated to sex.
The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.

Question 33

Most sex-linked human disorders are

Answer 33

due to recessive alleles and
seen mostly in males.
A male receiving a single X-linked recessive allele from his mother will have the disorder.
A female must receive the allele from both parents to be affected.

Question 34

Recessive and sex-linked human disorders include

Answer 34

hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting,
red-green color blindness, a malfunction of light-sensitive cells in the eyes, and
Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination.