Patterns of Inheritance

Chapter 9

Gregor Mendel

• Deduced the fundamental principles of genetics

• Cross-fertilization

What’s with the Peas?

• Little spontaneous variation between generations

• Can self-fertilize• Easy to control pollination• Possessed several easily

observable traits– Pea form– Pea color– Flower location– Flower color– Stem size

What do we get??

• Genetic cross– Bb x Bb

• P generation

• F1 generation

• F2 generation

Monohybrid Cross• Cross between parent plants that

differ in only one characteristic– Mendel developed four hypotheses

from the monohybrid cross:• There are alternative forms of genes

– Alleles

• For each characteristic, an organism inherits two alleles

– One from each parent

• Alleles can be dominant or recessive• Gametes carry only one allele for each

inherited characteristic

Mendel’s Laws

• Genes

– Set of instructions that

determine characteristics of an

organism

– Segments of nucleic acid that

specifies a trait

– Found at designated place on

chromosomes

• Locus

– Not all copies of a gene are

identical

Mendel’s Laws

• Alternative forms of a gene lead to

the alternative form of a trait

– Alleles

• way of identifying the two members

of a gene pair which produce

opposite contrasting phenotypes

• Chromosomes that are homologous

are members of a pair and carry

genes for the same traits in the same

order

Genes v. alleles

• Genes– Basic instruction

– Sequence of DNA

– General

– Hair color

• Alleles– Variations of that

instruction

– Specifics

– Brown hair

Genotype verse Phenotype

• Genotype – the alleles an individual receives at fertilization

• Homozygous – an organism has two identical alleles at a gene locus

• Heterozygous – an organism has two different alleles at a gene locus

• Phenotype – the physical appearance of the individual

Describing GenotypesHomozygous Dominant

when both alleles are dominantBB

Homozygous Recessivewhen both alleles are recessivebb

Heterozygouswhen one allele is dominant and one is recessiveBb

Punnet Square…..

Genetic cross determines arrangement

Pedigree Chart

Phenotypes are not always a direct translation of genotype

Phenotypes may also be influenced by the environment

Examples?• skin color influenced by sun• height/weight influenced by nutrition• animal coat influenced by climate

Remember…..

P = G + EP = G + E

Types of Phenotypic Traits

• Discrete traits

• Quantitative traits

1) Discrete Traits: determined by the action of a single gene

Only a few distinct categories exist for trait

Types of Phenotypic Traits

Shape of human hairline is a discrete trait

There are 2 alleles and 2 varieties:

Widow’s Peak = dominant

No Widow’s Peak = recessive

Human earlobe type is a discrete trait

There are 2 alleles and 2 varieties:

Attached earlobe = recessive

Free earlobe = dominant

Types of Phenotypic Traits

2) Quantitative Traits: determined by 2 or more genes

Has a range of phenotypes for that trait

Height is a quantitative traitThere’s a range of possible values

Others: weight, skin color

Past the Peas:Types of Dominance

1) Complete Dominance: 1 or other

2) Incomplete Dominance: 3rd effect

3) Codominance: some of both

Complete Dominance

Dominant Allele is always expressed when present

Recessive Allele is only expressed as homozygote

Incomplete Dominance

Alleles have combined (equal) effect on phenotype of heterozygote

Phenotype is intermediate

Codominance

Both alleles are visible in the phenotype of the heterozygote

Multiple Allelism: existence of more than 2 alleles of gene

Example: Blood type (A, B, O)

RememberRemember: Each person still only has 2 alleles for that trait, but more than 2 exist

Multiple Allelism: Blood typingABO Blood Type in Humans exhibits multiple allelism

Phenotype Genotype

O OO

A AA or AO

B BB or BO

AB AB *

How many ALLELES are there?

How many Phenotypes are there?

How many Genotypes are there?

3 ( A, B, O)

4 (A, B, AB, O)

6

What is the relationship between A and B alleles? codominance

Question: If a woman with blood type O marries a man with blood type B, can they have a child with

blood type A?Phenotype Genotype

O OO

A AA or AO

B BB or BO

AB AB*

No. The mother’s genotype must be OO and the father’s either BB or BO. Their child will either be type B (BO) or type O (OO)

Sex-linked Traits

Female XX Male XY

Genes located on the X or Y chromosome are sex-linked

X and Y chromosomes are not homologous, they contain different genes

Sex-linked traits

• Sex chromosomes– Are designated X and Y

– Determine an individual’s sex

– Influence the inheritance of certain traits

• Sex-linked genes– Are any genes located on a sex

chromosome

Sex-Linked Traits

Females (XX) have 2 copies of each gene on the X chromosome

Males (XY) have only 1 copy of each gene on the X chromosome

Males ALWAYS show X-linked alleles, regardless of dominance

Females can show a dominant condition if present on 1 or both X chromosomes

Females can only show a recessive condition if present on both X chromosomes

Sex-Linked Disorders in Humans

• number of human conditions result from sex-linked (X-linked) genes

• Red-green color blindness– characterized by a

malfunction of light-sensitive cells in the eyes

Question…..

• Will a mother that is colorblind automatically have a son that is colorblind?

Question…..

• Will a mother that is colorblind automatically have a son that is colorblind?

Yes!!!!!

Beyond Simple Inheritance Patterns

• Polygenic Inheritance – Occurs when a trait is

governed by two or more sets of alleles

Mutations

• Changes to the nucleotide sequence of the genetic material of an organism

• Can be caused by:– copying errors in the genetic

material during cell division– exposure to UV light or chemical

mutagens– Viruses– can be induced by the organism

itself

• Create variety within gene pool• Less favorable verse more favorable

Recessive Disorders

• Most human genetic disorders are recessive– Individuals can be

carriers of these diseases

Dominant Disorders

• Some human genetic disorders are dominant– Achondroplasia is a

form of dwarfism

– Huntington's disease