Chapter 13

BiologySixth Edition

Raven/Johnson

(c) The McGraw-Hill Companies, Inc.

Although people within families often resemble their parents, they all have a different appearance.

Traits are passed along generations:

Offspring

Father Mother

Offspring

Father Mother

Offspring First Generation (F1)

Parental (P) Generation

Second Generation (F2)

Many traits have only two types. For example, your earlobes are attached or not, flowers for some pea plants are purple or white.

Mendel crossed true-breeding purple flowers with true-breeding white flowers (Parental generation).

Every plant from the first generation (F1) had purple flowers

He then crossed F1 plants with each other. About 75% of the F2 flowers were purple and about 25% were white. He concluded that the trait for white flowers must have been present in the F2 generation – just not expressed.

It appeared that among the traits Mendel looked at, there was a dominant form and a recessive form.

Today we know that alleles occur at the same loci (position) on a chromosome.

(loci – plural, locus – singular)

The alleles segregate during the formation of the gametes (sperm or egg formation; meiosis) and each gamete has only one allele from each pair.

A capital letter indicates a dominant allele, which is expressed when present. A lowercase letter indicates a recessive allele, which is expressed only in the absence of a dominant allele.

Genotype refers to the genes of an individual which can be represented by two letters or by a short descriptive phrase.

Homozygous means that both alleles are the same; for example, AA stands for homozygous dominant and aa stands for homozygous recessive.

Heterozygous means that the members of the allelic pair are different—for example, Aa.

AA = Homozygous dominant

Aa = Heterozygous

aa = Homozygous recessive

Expresses dominant phenotype

Expresses recessive phenotype

3 genotypes 2 phenotypes

Dominant A allele for enzyme 1, dominant allele B for enzyme 2

Both A and B are needed to get purple.

Starting molecule Intermediate Anthocyanin(Colorless)(Colorless) (Purple)

EA EB

Dominant E allele for dark fur pigment

Dominant B allele for black nose, lips, and eye rims

- Deterioration of CNS

Enough protein to prevent deterioration of CNS

This disease is expressed as a homozygous recessive trait.

Progressive deterioration of brain cells.

This disease is expressed as a homozygous dominant trait.

IA – A antigen

IB – B antigen

i – no antigen

Sex-linked inheritance

23 types of chromosomes = 46 total chromosomes

22 autosomal chromosomes = 44 total autosomal chromosomes

1 sex chromosome = 2 total sex chromosomes (XY)

XX = female

XY = male

Genetic Recombination – crossing over

Crossing over is a physical exchange of chromosome material.

A B C D

a b c d

a b c D

A B C d

Crossing over is more common for genes that are far apart.

Given the frequency of crossing over, the relative distance between genes can be determined and a genetic ‘map’ can be created.

Created by the random inactivation of one of the X chromosomes in females early in development.

O = orange; o = black (white is regulated by a separate gene).

XOXo depending on which cell chromosome becomes a barr body, that cell early in development will produce either orange or black fur.

Trisomy 21 – caused by an extra chromosome 21.

Nondisjunction occurs when: 1.) both members of a homologous pair go

into the same daughter cell or

2.) when sister chromatids fail to separate and both daughter chromosomes go into the same gamete.

The result is a trisomy or a monosomy.

trisomy monosomy

Nondisjunction in meiosis I

monosomy

trisomy

Nondisjunction in meiosis II

Genetic counseling – identifying parents at risk of producing children with genetic defects and off assessing the genetic state of early embryos.

‘Restriction length fragment polymorphism’

Restriction nuclease cuts the DNA into fragments.

Mutated genes will have different size fragments than normal genes.

The End.