Biology 250Mendelian Genetics

It’s all about Gregor Mendel

The History of Genetics

• Genetics - The field of biology devoted to understanding how characteristics are transmitted from parents to offspring.

• Believe it or Not! Genetics began with the work of an Austrian monk named Gregor Mendel (now known as the “father” of genetics!)

The History of Genetics (continued)

• Some important facts about Gregor Mendel:– His task at the monastery was to tend the gardens. (This

is where he was first introduced to his favorite test subject: peas)

– He had taken courses in both science and mathematics including statistics. (This enabled him to see patterns in his experimental results which would otherwise have gone unnoticed.)

• Genetics gave rise to heredity – the study of the transmission of characteristics from parents to offspring

Mendel’s Experiment

• Mendel Experimented with pea plants. Qualities of the pea plant which made it useful for this purpose:– Has 7 characteristics which occur in 2 contrasting

traits. (purple OR white flowers for example)– Pea plants can either self-pollinate (fertilize

themselves) or cross-pollinate (fertilize each other)

Mendel’s Experiment (continued)

• Plant Terminology:– Pure – Plants which are pure for a specific trait will

always produce offspring with that trait. (Example: A pure purple flowering plant will always produce seeds which produce purple flowering plants.)

– Strain – A group of plants which are pure for a specific trait

Mendel’s Experiment (continued)

• Terms dealing with generations:– P1 (Parental Generation) – the starting generation of

the experiment.– F1 (First Filial Generation) – the generation produced

by crossing the parental generation. (Example: If your parents are the P1 generation, you are the F1 generation)

– F2 (Second Filial Generation ) – the generation produced by the F1 generation (Example: if your parents are the P1 and you are the F1 then your children will be the F2)

Vocabulary Review!Genetics

Heredity

Pure

Strain

P1 Generation

F1 Generation

F2 Generation

How are these two terms similar/related?

Mendel’s Experiment (continued)

• Mendel allowed pea plants to self-pollinated until they were pure for a specific trait. These plants were his P1 Generation.

• He crossed the members of his P1 Generation (white flower plant with purple flower plant) to produce the F1 generation

• He allowed the F1 generation to self-pollinated to produce the F2 generation.

Mendel’s Results & Conclusions

• Mendel concluded that each trait was controlled by a pair of alleles – alternative form of a gene– Dominant Allele – Must be expressed when

present. Will mask the appearance of other alleles.

– Recessive Allele – Will only show if no dominant allele is present.

Alleles

• Dominant alleles are abbreviated with capital letters (A, B, D)

• Recessive alleles are abbreviated with lower case letters (a, b, d)

• Each trait will have TWO alleles. (AA, Aa, aa)• If an individual has two dominant alleles for a gene (AA)

they are said to be HOMOZYGOUS DOMINANT• If an individual has two recessive alleles for a gene (aa) they

are said to be HOMOZYGOUS RECESSIVE• If an individual has one dominant and one recessive allele

for a gene they are said to be HETEROZYGOUS

Practice Practice Practice

In peas, purple is dominant to white so:R = purpler = white

RR = ?

rr = ?

Rr = ?

Genotype - The genes an organism has. (AA, Aa, aa)Phenotype – The physical appearance (purple or white flowers)

Probability

• The likelihood that a specific event will occur.• Can be expressed as a decimal, fraction, or

percentage.– 0.75– 75%– ¾

Predicting the results of crossesPunnett Square – Diagram used to predict the probability of a specific genotype or phenotype from a cross between two individuals.

Does NOT show you the actual offspring. It shows the likelihood of having a specific type of offspring.

Let’s try some Punnett Squares!

Reporting results of Punnett Squares

• If you are asked for the genotypic ratio you need to give the ratio of the genotypes.– 1AA: 2Aa: 1aa

• If you are asked for the phenotypic ratio you need to give the ratio of the phenotypes– 3 purple : 1 white

Performing a Testcross

• We will always know the genotype of an individual with the recessive phenotype.– Why?

• To determine the genotype of an organism with the dominant phenotype, we perform a testcross.

• In a testcross you cross the organism of unknown genotype with a homozygous recessive individual.

If A= purple and a = whiteAA = purpleAa = purpleaa = whiteThe only way to get the recessive phenotype is to have the aa genotype!

Testcross continued…

• How will this help us determine the genotype?– If the unknown individual is homozygous

dominant then….– If the unknown individual is heterozygous then….

All offspring show the dominant phenotype!

Half the offspring will have the dominant phenotype and half will have the recessive phenotype!

Let’s practice a testcross!

Dihybrid Crosses

• Sometimes you want to cross two individuals while looking at TWO different traits (flower color AND seed texture).

• Flower color: A = purple a = white• Seed texture: R = smooth r = wrinkled• Write the genotype of an individual who is heterozygous

for flower color and homozygous dominant for seed texture.

• Write the genotype of an individual who is homozygous recessive for flower color and heterozygous for seed texture.

AaRR

aaRr

Let’s see how it’s done!

Special Types of Inheritance

• We’ve been working with complete dominance – when one allele is completely dominant over the other.

• Sometimes alleles show incomplete dominance – when both alleles influence the phenotype of a heterozygote and the phenotype is a blend.

Example of Incomplete Dominance

• Four o’clock flowers show incomplete dominance.

• R = red• r = white• Rr = pink

Special Types of Inheritance cont.

• Codominance – When neither allele is dominant or recessive to the other and BOTH are expressed in a heterozygote.

• R = red• R’ = white• RR’ = red and white• Example: The roan horse.