The Basic Principles of Heredity

Question 1

Locus (loci plural)

Answer 1

The site a gene occupies in the chromosome.

Question 2

Alleles

Answer 2

Different form of a particular gene are alleles; they occuoy corresponding lovi on homologous chromosomes.

Question 3

Homozygous

Answer 3

An individual that carries two identical alleles is a homozygous for that locus.

Question 4

Heterozygous

Answer 4

If the two alleles are different in a locus the individual is heterozygous.

Question 5

Dominant allele

Answer 5

One allele, the dominant one, may mask the expression of the other allelel or the recessive allele in a heterozygous individual.

Question 6

Phenotype and genotype

Answer 6

Two individuals with the same phenotype may differ from each other in their genetic makeup or genotype.

Question 7

Principle of segregation

Answer 7

During meiosis the alleles for each locus separate or segregate from each other. When haploid gametes are formed, each contains only one allele for each locus. It is a direct result of homologous chromosomes separating during meiosis.

Question 8

Principle of independent assortment

Answer 8

Alleles of different loci are distributed randomly into gametes. The result can be genetic recombination, the production of new allele combinations that were not present in the parental generation.
It occurs because there are two ways in which two pairs of homologous chromosomes can be arranged at metaphase I of meiosis; the orientation on the metaphase plate determines the way chromosomes are distributed into haploid cells.

Question 9

Monohybrid, dihybrid and test crosses.

Answer 9

A cross between homozygous parents that differ from each other with respect to their alleles at one locus is called a monohybrid cross; if they differ at two loci it is called a dihybrid cross.
A test cross is a cross between an individual of unknown genotype and a recessive individual.

Question 10

Product rule

Answer 10

The probability of two independent events occrurring together can be calculated by multiplying the probabilities of each event ocurring separately.

Question 11

Sum rule

Answer 11

The probability of an outcome that can be obtained in more than one way can be calculated by adding the separate probabilities.

Question 12

Linkage

Answer 12

It is the tendency for a group of genes on the same chromosome to be inherited together. Independent assortment does not apply in this case.
Recombination of linked genes can result from crossing-over in meiotic prophase I (recombination also occurs from independent assortment).

Question 13

Y chromosome

Answer 13

It determines sex in mammals. The X chromosome contains many important genes unrelated to sex determination that are required by both males and females. A male recives all his X-linked genes from his mother while a female receives them from both parents.

Question 14

Pleiotropy

Answer 14

It is the ability of one gene to have several effects on different characters. Alternatively, alleles of many loci may interact to affect the phenotypic expression of a single character.

Question 15

Incomplete dominance

Answer 15

The heterozygote is intermediate in phenotype.

Question 16

Codominance

Answer 16

The heterozygote simultaneously expresses the phenotypes of both homozygotes.

Question 17

Multiple alleles

Answer 17

Three or more alleles that can potentially occupy a particular locus may exist in a population. A diploid individual has any two of the alleles; a haploid indivudal or gamete has only one.

Question 18

Epistasis

Answer 18

An allele of one locus can mask the expression of alleles of different locus.

Question 19

Polygenic inheritance

Answer 19

Multiple independent pairs of genes may have similar and additive effects on the phenotype.

Question 20

Norm of reaction

Answer 20

The range of phenotypic possibilities that can develop from a single genotype under different environmental conditions.