Intro to Genetics

Human Chromosomes

Mendel’s Work

Gregor Mendel- b.1822- Father of Genetics

Studied 7 different traits: seed shape & color, seed coat color, pod shape & color, flower position and plant height.

Pollination- transfer of pollen (carries sperm) to female part of flower (pistil) Self- pollen to same plant= 1 parent Cross- pollen to different plant= 2 parents

7 Traits

Mendel’s Work

Mendel cross pollinated pure breeding pea plants with opposing alleles.

EX: P (parent) generation= tall (TT) & short (tt)

F1 cross= tall (TT) X short(tt)F1 (first filial) Generation = all tall (Tt)

F2 cross= tall (Tt) X tall (Tt)F2 Generation = ¾ tall (TT or Tt) ¼ short (tt)

Mendel’s Work

From his results he concluded that: If an allele is expressed (shows up) in a hybrid, then

the allele is dominant(T). If an allele is NOT expressed in a hybrid, then the

allele is recessive(t).Phenotype- traits that show up in the organism.Genotype- genetic makeup (allele combination)

of the organism1. Homozygous (pure)- both alleles the same EX: TT

or tt2. Heterozygous (hybrid)- alleles differ EX: Tt

Mendel’s Work

Mendel’s Principles:1. Genes determine biological

characteristics.2. Offspring receive one allele for a gene

from each parent.3. One allele may be dominant over

another.4. Some genes segregate independently.

Independent Assortment

Probabilities

The likelihood that one trait or another will show up in offspring.

EX: ¼ BB, ½ Bb, ¼ bb25% BB, 50% Bb, 25% bb1 in 4 BB, 1 in 2 Bb, 1 in 4 bb1:2:1

Incomplete Dominance

Occurs when the alleles for a gene are neither dominant nor recessive.

Heterozygous organisms display a blend of both traits.

EX: Snapdragons flower color- RR= red rr= white Rr= pink

Codominance

Occurs when both alleles for a gene are dominant.

Both traits are expressed in the organism.

EX: Blood type in humans- AA= type A, BB= type B, AB= type AB

Multiple Alleles

Occurs when a gene has more than two alleles.Produces a variety of different phenotypes.

EX: Blood type in humans- three alleles= A, B, & O.

Type A= AA or AOType B= BB or BOType O= OOType AB= ABNote: both A and B alleles are dominant over O.

Polygenic Traits

Occurs when more than one gene controls an expressed trait in an organism.

More genes involved= wider range of phenotypes.

EX: Height, weight, metabolism, & skin color.