Genetics: A Monk a Pea and a Fly

Mendel and His Peas

• Born in Czech Republic in 1822

• Studied math in Vienna

• Was in charge of the monastery garden

• Conducted controlled breeding between true-breeding pea plants to study inheritance

Garden Peas : Tasty and Educational

• Easy to grow• Fast growing• Number of easily

recognized traits• Lots of offspring

(each seed)• Easy to control

crossing

Mendel’s Crosses

Dominant and Recessive Traits

• Noticed traits in the F1 were not a blend of the parents but, instead favored one parent

• Trait that shows up the F1 generation is Dominant the trait that is masked is Recessive

• Today : Traits chemical factors = Genes

Forms of a trait = Alleles

The F1 Cross

• Mendel then crossed the F1 plants and the recessive trait reappeared!

• What happened?

*Segregation : Alleles on homologous chromosomes separate when sex cells are produced

Think about the logic here

Genotypes and Phenotypes

• Genotype : expression of alleles present

Dominant alleles : Capital first letter of the dominant trait

Recessive alleles : Lower case script of the first letter of the dominant trait

Homozygote : Two of the same allele

Heterozygote : Two different alleles• Phenotype : Actual visible trait

• Example : Flower color

Purple (P) is dominant to white (p)

Cross a pure breeding plant with purple flowers with a pure breeding plant with white flowers.

1) Determine genotypes

2) Segregate alleles

3) Perform cross

Probability and Genetics

• Chance of an event occurring : probability• Does not indicate something will happen

only the likelihood that it can happen• Allele combinations follow rules of

probability• Probabilities in genetics can be

determined using simple math or Punnett squares

Mr. Curry’s Never Miss, Easy, Solves Every Type of Genetics Problem Method

1. Define alleles

2. Define phenotypes associated with genotypes

3. Write out cross

4. Produce gametes

5. Perform cross (Punnett square)

6. Determine genotypic and phenotypic ratios of offspring

7. Answer the problem

Typical Monohybrid

• Homozygous Dominant X Homozygous Recessive

P= F1= F2=

Two Traits at Once : Dihybrid Cross

• How do you determine inheritance for two traits at once?

- Dihybrid Cross• Just remember : Segregation and

Independent Assortment (For now, these genes exist on separate chromosomes)

• In the fruit fly Drosophila melanogaster, vestigial wings and hairy body are produced by two recessive alleles carried on different chromosomes. The normal alleles, long wings and hairless body, are dominant. If a vestigial-winged, hairy male is crossed with a female homozygous for

• both of the normal traits, what would be the phenotypes and genotypes of their progeny? If the

• F1 generation was allowed to mate randomly among themselves, what phenotypes and genotypes would be expected among the F2’s, and in what proportions?

• In some breeds of dogs, a dominant allele controls the characteristic of barking while trailing. In

• these dogs, another, independent gene controls ear shape, erect ears being dominant over floppy ears.

• If a dog breeder wants to produce a true-breeding strain of floppy-eared dogs that bark on the trail,

• how should she proceed, knowing that the alleles for erect ears and silent trailing are present in her

• kennels?

• P cross = Homozygous Dominant X Homozygous Recessive

P= F1= F2=

Other Types of Inheritance

• Not all genes are dominant or recessive• Sometimes get both alleles, a modified

form or the dominant or something new altogether

Incomplete Dominance

• Two alleles produce three phenotypes• Third phenotype is a muted version of the

“dominant”• Example 4 O’clock flowers

RR = Red

RW= Pink

WW = White

Codominance

• When two alleles are both equally expressed in the heterozygote

• Example : ABO bloodtypes

IAIA, IAi =A type blood

IBIB, IBi = B type blood

IAIB = AB type blood

ii = O type blood

Sex Determination

• In all animals : Sex is determined by the combination of sex chromosomes (23rd pair in humans)

• XX = female• Xy = male

Polygenic Inheritance

• Traits are governed by more than one gene

• Most traits are polygenic• Each gene may have two or more alleles• Many genes can be codominant or

incompletely dominant• Heterozygotes are intermediate in

phenotype

• A recessive gene combination will effect the phenotype

Multiple Alleles

The Fruit Fly

Drosophila melanogaster Homo sapiens