GENES AND HEREDITY

Mendelian Genetics Part 1

Introduction

• The passing of traits from parents to offspring is called heredity.

• Your traits are a result of the interactions of the genes from both your parents (50% from mom, 50% from dad)

• There are over 8 x106 combinations from the 23 chromosomes inherited from each parent.

• Your makeup is one of over 6.4 x 1013 (64 trillion) combinations of your parents gametes.

Early Beliefs and Mendel

• Early beliefs on heredity were flawed in many ways.

• Gregor Mendel is known as the father of genetics because his experiments laid the foundation for the science of genetics.

• Mendel was an Austrian monk (1822-1884) who worked with garden peas.

Mendel cont’d

• Pea plants were ideal because they: 1) had many contrasting traits, 2) reproduced quickly and in large numbers, 3) self-fertilized as well as cross-fertilized.

• Mendel crossed pure-breeding plants that always produced the same type of offspring with other pure-breeding plants.

• He crossed traits like: tall plants x short plant, round seeds x wrinkled seeds, green seeds x yellow seeds and looked for a pattern in the offspring.

Mendel’s Experiments

• Prior to Mendel it was believed that the crossing of traits would create a blend.

• Mendel showed that this was not the case for when he crossed pure tall plants with pure short plants, all the offspring were tall.

• Mendel showed that one trait always dominated over another.

• He reasoned that things called “factors” control the traits of a plant.

Mendel’s Experiment cont’d

• These factors were later called genes.

• He also realized that there were alternate forms of genes called alleles.

• Smooth (S) and wrinkled (s) are alleles for seed shape and the smooth is dominant while wrinkled is recessive.

• Notice the upper case letter denotes the dominant allele.

Monohybrid Crosses

• When pure smoothplants (SS) are crossedwith pure wrinkled (ss)plants, all the firstgeneration plants (F1)were smooth (Ss).• The next year Mendelcrossed the F1 plantstogether to see if thewrinkled trait wouldreappear in the secondgeneration (F2)

The 3:1 Ratio

• Mendel recovered 7324seeds, 5474 weresmooth while 1850 werewrinkled.• The ratio 5474:1850 canbe simplified to 3:1.• The chart to the right iscalled a punnettsquare and explainshow these alleles couldform offspring in a 3:1ratio.

Summary of Mendel’s Work

• Traits are controlled by genes which occur in pairs. Different forms of a gene are called alleles.

• One allele can mask the expression of another which is called the principle of dominance (eg. S-smooth is dominant over s-wrinkled)

• Therefore SS is smooth, Ss is also smooth, while only ss is wrinkled.

• Pairs of alleles separate during sex cell formation so that each sex cell (sperm or egg) only has one member of each pair -- law of segregation.

Meiosis Supports Mendel

• 25 years after Mendel’s work, the study of meiosis in cells explains how genes segregate into sex cells or gametes.

• A hybrid parent Ss will produce 50% S gametes and 50% s gametes.

• Now mathematics can be applied in biology to solve heredity problems

---- Mendelian genetics!

Review

Why did Mendel choose pea plants?

• Pea plants have many contrasting traits, reproduce quickly with lots of offspring, and can be cross-fertilized.

What are alleles? Give some examples.

• Alleles are different versions of genes. E.g. Tall & short plants, brown & blue eyes

Review

At what stage of meiosis do pairs of alleles separate? (Mendel’s law of segregation)

• When homologous chromosomes separate during anaphase 1.

If you are hybrid for freckles (Ff), what % of your gametes will have the dominant freckles allele (F) ?

• 50%

ReviewWhat are the chances of a couple who are hybrid brown

eyed (Bb x Bb) of producing a child with blue eyes (bb)?

Bb x Bb

B b

B BB Bb

b Bb bb

there is a ¼ chance of producing blue-eyed child, or 25%