Biology Chapter 11

Basic Principles of Heredity

Gregor Mendel

A 19th century monk who began the scientific study of

inheritance through his breeding of pea plants

Heredity

transmission of genetic info from parent to

offspring, follows predictable patterns

Genetics

the science of heredity

Hybrid plants and animals are offspring of ___

two genetically dissimilar parents

All hybrid plants that are offspring of genetically pure parents are

similar in appearance

When hybrids mate with each other, they do not breed true because

offspring show a mixture of traits

phenotype

physical appearance of an organism

genotype

the genetic makeup

A true-breeding line produces only offspring that express

the same phenotype gen after gen

P generation

the parental generation are genetically pure individuals

F1 Generation

the first generation of offspring all looked alike and resembled one of the two parents

F2 Generation

the second generation resulted from a cross between F1 individuals or by self-pollination of F1 individuals

What does Mendel’s model show?

Inherited traits pass from parents to offspring as unmodified factors

What else does Mendel’s model show?

Each individual has two sets of factors, one of each pair inherited from the mother and one from the father.

Principle of segregation

The paired factors separate prior to the formation of reproductive cells

Although factors may be expressed or hidden in a given generation…

they are never lost

Principle of independent assortment

Each factor is passed to the next generation independently from all other factors

Genes

units of heredity that affect an organism’s traits

Explain genes in terms of how they are a DNA sequence in a chromosome

A DNA sequence in a chromosome that contains the information to make an RNA or protein product with a specific function

Locus

the site a gene occupies in the chromosome

Alleles

different forms of a particular gene which occupy corresponding loci on homologous chromosomes

Homozygous

two identical alleles

Heterozygous

two different alleles

During meiosis, alleles for each locus

segregate from one another

When haploid gametes are formed, each contains

one allele for each locus

Monohybrid cross

a cross between homozygous parents with different alleles

Dihybrid cross

parents differ at two loc

Test Cross

cross an individual of unknown genotype with a recessive individual to determine the unknown genotype

Why, with reference to alleles, can’t we always determine an organism’s alleles by its phenotype?

Because some alleles are dominant and others recessive

Alleles of different loci are distributed randomly into gametes during meiosis. What can this result in?

genetic recombination

Two pairs of alleles carried on non-homologous chromosomes are inherited

independently

Mendel’s segregation of alleles is a direct result of

homologous chromosomes separating during meiosis

Chromosome theory of inheritance

assumes genes are linearly arranged in specific locations along

chromosomes

In monohybrid crosses, expected ratio of dominant and recessive phenotypes is

3:1

What are the chances that a family with 2

children will have 1 boy and 1 girl? There are two

mutually exclusive possibilities:

1st girl 2nd boy- ¼
1st boy 2nd girl- ¼

When does independent assortment not apply?

if two loci are linked close together on the same pair of homologous

chromosomes

How many pairs of chromosomes do humans have

23

How many pairs of autosomes do humans have?

22

How many pairs of sex chromosomes do humans have?

1

What chromosomes do females have

2X

What chromosomes do males have

XY

Dosage compensation

a mechanism that allows males and females to produce the same amounts of proteins coded by X-linked genes

Dosage comp. usually involves random inactivation of

one of the two X chromosomes in the female

Barr body

a dense, metabolically inactive X chromosome

Variegation

different colored zones. patches of color

Incomplete dominance

the heterozygote is intermediate in phenotype

Co-dominance

the heterozygote simultaneously expresses phenotypes of both homozygotes

Pleiotropy

the ability of one gene to have several effects on different characters

Most cases of pleiotropy can be traced back to simple cause. What is an example of this?

a defective enzyme that affects the functioning of many types of cells

Epistasis

An allele of one locus can mask the expression of alleles of a different locus and express their own phenotype instead

Polygenic Inheritance/ polygenic traits

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

Give an example of polygenic inheritance

As many as 60 loci may contribute to inheritance of skin pigmentation in humans

How is polygenic inheritance characterized by in regards to the F1 generation?

it is intermediate between two completely homozygous parents

How is polygenic inheritance characterized by in regards to the F2 generation?

it shows wide variation between the two parental types

What does phenotypic expression depend on?

a combination of bio and environmental influences

How does the environment regulate activity of certain genes?

The environment turns genes on in some environmental conditions and off in others

Norm of reaction

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