Introduction to Mendelian Genetics

Learning goals

• Understand the basic terminology of mendelian genetics, not limited to (phenotype, genotype, homozygous, heterozygous, P,F1,F2 generation(s), dominant, recessive, alleles, gene

• Solve monohybrid problems using Punnett square method

Gregor Mendel (1822-1884)

• Used pea plants to study genetics, heredity and variation.

• Looked at 7 different traits that only had two variations (two different alleles)

• Discovered Mendel’s Laws of genetics

1919thth Century Britain Century Britain

• Before Mendel, from breeding ornamental plants, scientists believed in a blending theory of inheritance.

• Charles Darwin theorized that offspring have variations of their parents' characteristics, but he was unable to explain why (1870s).

Gregor Mendel (1822-1884)Gregor Mendel (1822-1884)

• Austrian monk

• Considered the father of genetics (now aka Mendelian Genetics)

• Why the pea plant?– grows easily and quickly – has seven traits that are easily

identified – it is easy to manipulate

7 traits Mendel looked at7 traits Mendel looked at

Mendel's Pea Plant ExperimentMendel's Pea Plant Experiment

• True breeding: Individuals that only contain one variation of a trait and therefore can only pass this one variation on to future generations. – AKA homozygous: having two alleles that are the same.

• Mendel selected true bred plants for different traits and cross-fertilized them to see what would happen.

• For example:– True bred Tall x true bred dwarf

• What do you think the offspring looked like?X

• 100% of the offspring plants were tall! (Instead of a blending – where all plants would be medium sized...)

X

x

• Mendel then let the F1 generation self-pollinate (fertilize themselves)

• F1 generation – (filial (son or daughter) generation) offspring from the cross (reproduction) of the Parent or P generation.

• this time most were tall, but some were short and ALWAYS 3:1 ratio

Why?Why?• Each plant in the F1 generation carried an allele from the

P generation; a tall allele and a dwarf allele. This is a hybrid.

• Hybrids: Individuals that contain more than one variation of a trait. Also known as heterozygous.

• Traits are dominant or recessive. If a dominant trait is inherited, it will be expressed.

• What is the dominant trait for height in pea plants?

Mendel’s “Key Terms”• Phenotype: the appearance of traits in an organism (eg.

short or tall)

• Genotype: the specific genes that an organism has (which alleles, dominant or recessive)

• Every individual has a phenotype and genotype for every trait.

Genotype vs. Phenotype

Which Allele is expressed?

Can you tell the genotype of an individual by their phenotype?

A flower species is dominant for the purple trait and recessive for the white trait. What would happen if we crossed two hybrid (heterozygous) parents? We use a Punnett square to find out.

Genotypic proportions

The genotypic proportions of the offspring from the cross of two heterozygous parents (Bb) are:

• BB __/4

• Bb __/4

• bb __/4

Phenotypic proportions

The phenotypic proportions of the offspring are:

• Purple __/4 or __%

• White __/4 or __%

Punnett Square

Monohybrid cross problem

Curly hair is dominant over straight hair. If a

mother is homozygous dominant for curly

hair and the father has straight hair, what

type of hair will the children have?

Test Crosses

• When geneticists want to know if an individual is heterozygous or homozygous– Why? For breeding purposes

• Cross between unknown phenotype and homozygous recessive individual– (Pp or PP) x pp

• Pp x pp = 50% purple, 50% white• PP x pp = 100% purple

Mendel's Laws and PrinciplesMendel's Laws and Principles

• Law of Segregation: when any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy of a gene and therefore only one allele for all possible traits.

Principle of Segregation Demystified

The principle of segregation is explained by the behavior of homologous chromosomes at meiosis.

Segregation

Law of Segregation

• Law of Independent Assortment: alleles of different genes assort independently of one another during gamete formation, so different traits are inherited independently of each other