Inheritance of Single-Gene Differences

I. Transmission genetics – link between meiosis & Mendel’s postulates

II. Mendel: father of genetics A. Mendel’s Empirical approachB. Contrasting characters

III. Monohybrid crossA. Mendel’s resultsB. Mendel’s First “law” equal segregationC. Punnent Square

IV. Dihybrid crossA. Mendel’s Second “law” independent assortmentB. Using the testcross

I. Transmission genetics – link between meiosis & Mendel’s postulates

The transmission of discrete units (genes located on chromosomes) from parent to offspring

Correlation between the behavior of chromosomes during meiosis and the transmission of traits

Terminology review• Genes come in different forms = ALLELES

• Phenotype = expressed form of a character (what an individual looks like)

• Genotype = specific set of alleles carried by an individual (the actual genetic composition)

• Homozygous = the alleles of a gene are identical (AA)• Heterozygous = the alleles of a gene are different (Aa)• Dominant allele = an allele that expresses its phenotypic

effect even when heterozygous… therefore AA and Aa have the same phenotype

• Recessive allele = An allele whose phenotypic effect is notexpressed in a heterozygote… therefore (a) can only be expressed when the individual is homozygous – (aa).

Genetic Crosses

• Self Cross =

• Haploid Cross = simplest, each gene present in 1 copy only (fungi)

• Diploid Cross = each gene present in 2 copies

II. Gregor Johann Mendel

“Father of Genetics”

Mendel’s success

• Came up with an elegant model of experimental design– chose a good “model” organism:

Pisum sativum– restricted his examination to one

or very few pairs of contrasting traits in each experiment

– took meticulous notes with accurate quantitative records

A. Mendel’s Empirical approach

Mendel’s experiments were designed to determine the quantitative relationships from which laws could be discovered

B. Contrasting characteristics of the garden pea

III. Monohybrid cross

• Hybridization = when two plants of the same species but with different characteristics are crossed (mated) to each other.

• Mono = dealing with one pair of contrasting characteristics

• P – • F1 – • F2 –

A. Mendel’s resultsParental F1 F2 F2 ratio

Round x wrinkled All round

5474 round 1850 wrinkled

2.96:1

Yellow x green seeds All yellow

6022 yellow 2001 green

3.01:1

Purple x white All purple

705 purple 224 white

3.15:1

Inflated x pinched All inflated

882 inflated 229 pinched

2.95:1

Green x yellow pods All green

428 green 152 yellow

2.82:1

Axial x terminal All axial 651 axial 207 terminal

3.14:1

Long x short All long 787 long 277 short

2.84:1

Mendel’s explanation

1) the existence of “factors” – particulate theory of inheritance

2) genes are in pairs,

3) the principle of segregation, 4) gametic content – the F2 3:1 ratio is based on a

1:1 segregation in a heterozygote5) random fertilization – gametes are brought

together for fertilization in a random manner

B. Mendel’s First Law• Equal Segregation = The two members of a

gene pair segregate from each other into the gametes; so half the gametes carry one member of the pair and the other half of the gametes carry the other member of the pair.

C. Using Punnett Squaresin Genetic Crosses

Punnett squares

– Considers only genes of interest

– List sperm genotypes across top

– List egg genotypes down side

– Fill in boxes with zygote genotypes

P p

FrequenciesFrequenciesPhenotypesPhenotypes

GenotypesGenotypes

FrequenciesFrequencies

Making a Punnett Square:Heterozygous X Heterozygous

Eggs of Heterozygous PlantEggs of Heterozygous Plant

Pollen ofPollen ofHeterozygous PlantHeterozygous Plant

1111 22

P

p pP

PpPP

pp

PP pppP Pp

IV. Dihybrid Cross

• Follows the inheritance of two different traits within the same individual.

A. Mendel’s Second Law

• Independent Assortment = two different genes will randomly assort their alleles during gamete formation

F1 cross: GgWw x GgWw

(Hair color) & (Hair length)Black/Brown Short/Long

P: Black, short x Brown, long

B. Using the testcross

Pedigree Analysis

How do doctors know if a trait is inherited?

• They take a “family history”, and show it in a diagram form known as a pedigree

Pedigree Symbols

Example 1:• Grandparents had two

children: a son and a daughter.

• Their son had the trait in question. He marries a woman without the trait.

• One of the son’s four children (a boy) had the trait.

Example 2:• Grandpa has the trait,

grandma doesn’t.• Of their five children,

one son and two daughters have trait. One son and one daughter don’t have trait.

• One daughter with trait marries man without. Of their five kids, one son and one daughter have trait.

Can you tell from a pedigree if a trait is dominant or recessive?

Could this trait be dominant?• If it’s dominant, the affected daughter would have to be either DD or Dd.

• If she’s DD or Dd, she would have had to get a dominant D allele from a parent.

• But, if were dominant and the parent had a D allele, the parent would have the trait, too.

• Therefore, trait is recessive, NOT dominant; daughter is dd

What are the genotypes?• Parents don’t have the

trait, so they can’t be dd.

• But, since they each passed a recessive d allele to daughter, they must each be Dd.

• Since the trait is recessive, daughter must be dd.

dd

Dd Dd

DD or Dd

Could this trait be dominant?• If it’s dominant, the affected

daughter would have to be either DD or Dd.

• If she’s DD or Dd, she would have had to get a dominant D allele from a parent.

• Dad has the trait, so he could have given her a D allele.

• So, trait is dominant.

What are the genotypes?• Since the trait is dominant,

mom can’t have a D, or she would show the trait.

• So, mom is dd.• The son doesn’t have trait, so

he must be dd, too.• If son is dd, he got one d from

mom; one from dad.• Dad must be Dd; daughter is

Dd.

ddDd

Dd dd