Lecture 2: Introduction to Mendelian Genetics

Mendelian Genetics

Gregor Mendel 1865

Key: Particulate Inheritance

• Discrete units of heredity (genes) that are inherited intact through the generations

Mendel's Experiments

• Used pure-breeding (genetically identical) lines of garden peas

• Examined crosses involving seven different characters

What Mendel Observed

Here we consider his crosses involving Seed color -- Yellow versus Green

• The F₁ were all Yellow

• However, Green Segregated out in F₂
  • Strong evidence for discrete units of heredity , as "green" unit obviously present in F₁, appears in F₂

  • 3:1 ratio of Yellow : Green in F₂

Mendel also found that Parental, F₁, and F₂ Yellow peas behaved quite differently

• When examined each F₂ yellow family separately, Mendel found
  • 2/3 of the F₂ yellows give 1/2 yellow, 1/2 green

  • 1/3 of F₂ yellows gave all yellow progeny
    

  Mendel' s explanation

  
  • Genetic information exists as discrete units occurring in pairs

  • YY is the genotype of the pure Yellow line

  • yy the genotype of pure Green line
    • Y dominant to y (y is recessive to Y)

    • YY, Yy (denoted by Y-) = Yellow

    • yy = green

  • Here, the discrete units segregate out in the F₂
  

  Genetic Notation

  • A LOCUS (or a GENE) is the region of DNA that codes for a particular trait
    • The Yellow locus

  • Different discrete units at a locus are called different ALLELES
    • Y is the yellow allele
    • y is the green allele

  • The trait that we observe is called the PHENOTYPE

  • The Genetic composition is called the GENOTYPE
    • A Yellow phenotype can result from either a YY or a Yy genotype

  • A HOMOZYGYOTE (or HOMOZYGOUS INIDIVIDUAL) has both copies of the allele the same
    • YY -- a Y homozygote
    • yy -- a y homozygote

  • A HETEROZYGOTE has two different alleles at a given locus
    • Yy is a heterozygote

  • An allele may be DOMINANT to another, where the heterozygote has the same value as the homozgyote
    Yy and YY have the same phenotype, Yellow. Hence, Y is dominant to y, in that the Yy heterozygote has the same value as the YY homozygote

  • We can also say that an allele is RECESSIVE to another
    y is recessive to Y

Crosses with Two traits

• Crossing pure-breeding lines differing in two characters

• Example: for Seed shape
  • RR, Rr = round

  • rr = wrinkled

• Cross yellow, wrinkled pure line with round green

The result is a 9 : 3 : 3 : 1 ratio of the 4 different phenotypes

In his crosses, Mendel reported Independent Assortment , where the R any Y allele assort independent of each other

Mendel cheated (or was very lucky)

The seven characters he used to show independent assortment were all on different chromosomes. If they were on the same chromosome they would NOT show independent assortment.

The Key Concepts of Mendelian Genetics

At any given gene, each parent has two copies (two alleles).

Each parent gives you one of their alleles at random

Example

• Your father has genotype Aa at the "A" gene and Bb at the "B" Gene

• Your mother has genotype aa at the "A" gene and Bb at the "B" gene

• What possible genotypes can you have at the "A" locus?
  • Half the time, dad gives you his A allele. Otherwise, half the time you get his a allele

  • You mom also gives you one of her alleles at random, but both her alleles are a, so she always gives an a

  • Hence, the possible genotypes are 50% Aa, 50% aa

The Genetics of ABO Blood Groups

The ABO blood group is commonly used in crime samples as an inital screen. It is also used as a simple paternity test

• There are three alleles at the ABO locus, I^A, I^B, and i.

• I^A, I^B are codominant, I^A and I^B are dominance over i

The Genetics of Rh Blood Groups

A second blood character is often used, whether you are Rh positive or RH negative

Rh system was named after rhesus monkeys, where the blood from Rh positive humans clumps when interacting with antiserum generated from rhesus blood.

Genetics: Rh negative occurs only in homozygote recessive individuals, with dd = Rh-, and DD, Dd = Rh+.

Example

Consider the offspring from a large family with an A+ father and an O negative mother. They have a large number of offspring:

1/4 are A-

1/4 are A+

1/4 are O-

1/4 are O+

What is the genotype of the mother and father?