April 2008

Mendelian GeneticsChapter 10.2

Genetics – the scientific study of heredity.– Studied for centuries

• Artificial selection (i.e. breeding)• Domestication

A trait is a characteristic that can be passed from parent to offspring.

Gregor Johann Mendel(1822-1884)

• 1822 - Born in Heizendorf, Austria

• 1843 - Enters monastery• Studied physics, math,

chemistry, botany, paleontology, & plant physiology

• Became a science teacher• 1860s – returns to

monastery – Studies inheritance of traits in

pea plants.– Uses 20,000 plants over 7

yearshttp://www.youtube.com/watch?v=0vAAf4g5iF8

Why Peas?• Mendel choose peas

because… … they are easy to grow. … they reproduce rapidly.… they were readily

available.… the structure of pea flower

enabled him to control fertilization.

… they possessed distinctive traits that could be easily distinguished.

The Sex Lives of Peas

The Sex Lives of Peas

• Pea plants usually reproduce by self-fertilization.– leads to “true-breeding” or

“pure-breeding” plants• Mendel could easily cross-

pollinate pea plants by hand.– can result in “hybrids”

Pea flower

Mendel’s Experimentx

Mendel’s Conclusions1. Each trait is controlled by a ‘factor.’

– since there were two distinct traits possible, there must be two distinct factors

– ‘factor’ is called an allele – alternative forms of a single gene, or region of a chromosome that codes for a trait.

2. Alleles are paired in each plant.3. If an organism has two different alleles for

a trait, only one is shown.– dominant allele – fully shown (expressed)– recessive allele – masked by the dominant

allele

Law of DominanceIf an organism has two different alleles for a trait, only one is shown.

– dominant allele – fully shown (expressed)– recessive allele – masked by the dominant allele

Dominant allele

YRecessive

alleley(expressed with a capital

letter)(expressed with a lower case letter)

Allele NotationOrganisms have two alleles for each trait

– Remember, the alleles are found on homologous chromosome pairs

Homozygous – organism with two of the same alleles for a particular trait.(i.e. YY or yy)

Heterozygous – organism with two different alleles for a particular trait.(i.e. Yy)

YY yy

Yy

Genotypes“types of genes”

Phenotypes(outward expression)

Law of SegregationThe two alleles for each trait separate

during meiosis.

Beginner’s Guide - http://www.youtube.com/watch?v=Y1PCwxUDTl8

Simulating the Law of SegregationGroup Yellow Blue

123456

TOTALS

Ratio of yellow offspring to blue offspring

_____ yellow: _____ blue

ReferencesBiggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007. Fairholm, Rebsie. "Daughter of the Soil." [Weblog How to breed your own garden peas] 20

May 2007. 2 Apr 2008 <http://daughterofthesoil.blogspot.com/2007/05/how-to-breed-your-own-garden-peas.html>.

Drill – March 10, 2009Which of the following genotypes are homozygous? heterozygous?Aa BB cc Dd ee Ff GG

In peas, purple flower color is dominant to white flower color.

• What would be the genotype of a plant with white flowers?• What would be the phenotype of a plant with the genotype ‘Pp’?

Which one of Mendel’s Laws explains why some traits in pea plants appear to ‘skip’ generations? Explain the law.

Mendel’s Experimentx

Using Mendel’s Laws to Predict• Geneticists can use Mendel’s Laws and the rules

of probability to predict whether an allele (or alleles) will be passed from parent to offspring.

probability (n.) – the likelihood that an event will occur

• Probability can be expressed as…• a fraction (1/2),• a percentage (50%),• or a ratio (1:1).

Making Predictions - Dimples

Problem:Having dimples (depressions in the cheek when you smile) is a dominant trait controlled by a single gene.

Two parents have dimples and are heterozygous for the trait. What is the probability that they will have a child with dimples?

Making Predictions - Dimples

Solution:From the information in the problem you know that both parents have the heterozygous genotype ‘Dd’.

D = dominant allele (dimples)d = recessive allele (no dimples)

From here there are two ways to solve the problem – using basic probability or using a Punnett square.The Punnett square, developed by Sir Reginald Punnett in the early 1900s, is pictured at right.

D d

D DD Dd

d Dd dd

♂ male gamete types

♀ fe

mal

e ga

met

e ty

pes

A Word About RatiosA cross that involves two hybrids (heterozygous parents) for a single trait is called a monohybrid cross.

There are always three possible genotypes in a ratio of 1:2:1.

There are two possible phenotypes in a ratio of 3:1.

D d

D DD Dd

d Dd dd

♂ male gamete types

♀ fe

mal

e ga

met

e ty

pes

Pea Plants Provide Practice Problems

1. In peas, round-shaped peas are dominant to wrinkled peas. A farmer takes a pea plant that is heterozygous for round peas and breeds it with a pea plant that produces wrinkled peas. What is the probability that the offspring will produce round-shaped peas?

2. Tall pea plants are dominant to short pea plants. Two heterozygous tall pea plants are bred together. What is the probability that the offspring will be heterozygous like their parents?

3. Green pea pods are dominant to yellow pea pods. A farmer has two plants with green pods. He crosses them in the hopes of getting some plants with yellow pods? Is this possible? Explain our answer.

ReferencesBiggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.Szeto, C. “Asian (Oriental) American girl with dimples” Wikimedia Commons. Jan 21,

2008. Retrieved Apr 6, 2008 from <http://commons.wikimedia.org/wiki/Image:Asian_girl_with_dimples.jpg>.