5.1 Mendelian Inheritance

Pages 184-193

Trait

A particular version of a characteristic that can be inherited.Example: eye colour

Gregor Mendel

The father of genetics (1822-1884)

Pea plants

Experiments

Flower colourFlower positionStem lengthSeed shapeSeed colourPod shapePod colour

True-breeding plants

Plants that produce offspring that have the same trait as the parent.

hybrid

• Offspring of two different true-breeding plants.

Monohybrid crosses

breeding two organisms with different versions of only one traitP generation (parental) …true breedingF1 generation (filial generation)…hybrid offspring

Purple flower crossed with white flower

F2 generation

Crossing the F1 generation: F2 generation with some purple and some white, but no pink flowers.3:1 ratio of purple to white flowers

Mendel’s conclusions

1. For each trait, an organism carries two genes, one from each parent.

2. Parents donate only one copy of the gene to their offspring. (sperm and eggs are haploid)

Allele

A specific version of a trait or form of a gene.

Genotype vs. Phenotype

Genotype: the organism’s genes (genetic make-up) including genes that are hidden from view

Phenotype: the traits that are expressed, the physical appearance of the organism.

Dominant vs. Recessive Alleles

A Dominant Allele is expressed if it is present in the gene. Ex. PP, Pp, pP

A Recessive Allele is expressed only if there is no dominant gene Ex. pp

Purple vs. White flowers

• Let P be the purple flower gene• Let p be the white flower gene• Homozygous…both alleles are the

same, PP or pp• Heterozygous…both alleles are

different, Pp

What you see …isn’t always what you get!

Genotype PhenotypePP homozygous dominant

purple

Pp heterozygous Purple (carrier of white gene)

pp homozygous recessive

white

Punnett Square

Punnett Square

A grid system for predicting the probability of inheriting certain genes

Probability

The measure of the chance that an event will happen.Example: Toss a coin, call it.The chance of tails is 50%

Test Crosses

To determine if the parent is PP or Pp, cross it with a homozygous recessive individual.

If 100% of the offspring show the dominant trait the parent was PP.

If the offspring displays both phenotypes, the parent was Pp.

Is the parent PP or Pp?

Eye colour

B…brownb…blue

Green pods vs. yellow pods

Y…yellow (dominant)y…green (recessive)Cross between yellow heterozygous and green homozygous recessive