Human Genetic Analysis

Chapter 11

Complex inheritance of traits does not follow inheritance patterns described by Mendel.

Several genes can influence a trait- polygenic trait.

Determining the effect of these genes is difficult.

Example: A horse with red hair mates with a horse with white hair, and their offspring has both red and white hair.

How can this be?

Codominance

When two dominant alleles are expressed at the same time, both forms of the trait are displayed.

Different from incomplete dominance because both traits are displayed.

Example: red flower x white flower = red flower x white flower =

red and white flowered offspringred and white flowered offspring equal number of red and white flowers

Example: Black horse x white horse = roan coatBlack horse x white horse = roan coatequal number of black and white hairs

Intermediate Traits

Incomplete dominance - an individual displays a trait that is intermediate between the two parents.

Example: red flower x white flower = pink flowerred flower x white flower = pink flower Neither the red nor the white allele is completely dominant

Multiple Alleles

X-Linked Traits

The trait is carried by females on the X chromosome through a recessive allele.

Males will have the disorder because they only have one X chromosome.

Traits influenced by the Environment

An individual’s phenotype often depends on conditions in the environment.

Example: fur color in Siamese catsDark fur around cooler parts of the body- ears, nose,

paws, and tails.

Example: skin tone in humansExposure to the sun alters the color of the skin.

MutationsChanges in genetic material.

The harmful effects produced by inherited mutations are called genetic disorders.

Many mutations are carried by recessive alleles in heterozygous individuals. ““carrierscarriers””

Genetic Counseling

o Genetic counseling is a form of medical guidance that informs people about genetic problems that could affect them or their offspring.

1. Genetic AbnormalityRare, uncommon version of a trait.

2. Genetic DisorderAn inherited condition that sooner or later will cause mild to severe medical problems.

3. SyndromeA recognized set of symptoms that characterize a given disorder.

4. DiseaseIllness caused by infectious, dietary, or environmental factors, NOT by inheritance of mutant genes.

Disorder Mutation Chromosome

Color blindness P X

Cystic fibrosis P 7q

Down syndrome C 21

Hemophilia P X

Klinefelter syndrome C X

Sickle-cell disease P 11p

Tay–Sachs disease P 15

P – Point mutation, or any insertion/deletion entirely inside one geneD – Deletion of a gene or genesC – Whole chromosome extra, missing, or bothT –Trinucleotide repeat disorders: gene is extended in length

Karyotypes

Karyotypes

micrograph in which the pairs of homologous chromosomes are arranged in decreasing size

22 autosomes are matched together with one pair of nonmatching sex chromosomes

Used to study chromosomes in hopes of identifying genetic abnormalities or disorders.

PEDIGREE CHARTS

© 2007 Paul Billiet ODWS

What is a pedigree chart?

a chart of the genetic history of a family over several generations.

used to study the transmission of hereditary conditions

© 2007 Paul Billiet ODWS

Constructing a Pedigree

Female

Male

Connecting Pedigree Symbols

Married Couple

Children

Symbols used in pedigree charts

Affected

X-linked

Carrier

Deceased

© 2007 Paul Billiet ODWS

. .

Symbols used in pedigree charts

Normal male Affected male Normal female Affected female

© 2007 Paul Billiet ODWS

Organizing the pedigree chart

A pedigree chart of a family showing 20 individuals

© 2007 Paul Billiet ODWS

Organising the pedigree chart

Generations are identified by Roman numerals.

I

II

III

IV

© 2007 Paul Billiet ODWS

Organising the pedigree chart Individuals in each generation are identified by Arabic numerals

numbered from the left Therefore the affected individuals are II3, IV2 and IV3

I

II

III

IV

© 2007 Paul Billiet ODWS

Interpreting a Pedigree Chart

1. Determine if the pedigree chart shows an autosomal or X-linked disease.

If most of the males in the pedigree are affected, then the disorder is X-linked.

If it is a 50/50 ratio between men and women the disorder is autosomal.

Example of Pedigree Charts

Is it Autosomal or X-linked?

Answer

Autosomal

2. Determine whether the disorder is dominant or recessive.

If the disorder is dominant, one of the parents must have the disorder.

If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.

Example of Pedigree Charts Dominant or Recessive?

Answer

Dominant

Example of Pedigree Charts Dominant or Recessive?

Answer

Recessive