Gregor Mendel

Pea Plants and Inheritance Patterns

• Mendel was born in 1822 in Austria• His father was a peasant farmer,

tenanted to a local aristocrat who was very interested in scientific crop improvement

• The family was very poor• Mendel’s early education was with local

priest and teacher• Showed considerable academic

capability and so was sent to larger town with more opportunities

• Financial problems plagued him, affecting his health, so Mendel decided to enter monastery as means of support

Who is Gregor Mendel?

At the Monastery…

Mendel

At the Monastery…

• Mendel taught 6th and 7th grade age children

• He also had access to the monastery’s library

• Fr. Napp, head of the monastery, had a shared interest in botany and agriculture

Mendel’s Garden

Mendel’s Model Organism –

The Garden Pea

1. Self-Fertilizing2. Matured Quickly3. Several Easily

Identifiable Traits

Used Consistent Methods:

Opened flower & placedpollen from one type ontothe stigma

• Seed Shape• Seed Color• Pod Shape• Pod Color• Flower Color• Flower Position• Stem Length

Pea Plant Characteristics

Mendel’s First Experiment

• Crossed Pure Tall Pea Plant (TT) x Pure Short (Dwarf) Pea Plant (tt)

• Hypothesis:– The offspring would be:

• All tall• All short• All intermediate• Some would be talls and some short

The Results?

• 1st Experiment: – Crossed Pure Tall x Pure

Short– All offspring (F1) tall

• 2nd Experiment: – Bred F1– Ratio of 787 tall to 277

short (3:1)• Similar to chance events

from flipping 2 pairs of coins

A cross between individuals differing in single character is a monohybrid cross.

The analysis of monohybrid crosses allowed Mendel to deduce the Law of Segregation…

Genes come in pairs that separate in the formation of sex cells (and these sex cells unite randomly at fertilization).

Monohybrid Crosses and the Law of Segregation

Staying the Course –Crosses to the F2 (The Grandchildren)

The reappearance of the recessive trait in ¼ of the F2, suggests genes come in pairs that separate in the formation of sex cells.

• Crossed one of the F1 tall plants with its dwarf parent: F1 Tall x Dwarf

• Possible Outcomes:– All would be tall– Mixture of Tall & Dwarf– All would be intermediate

• Experimental results

Therefore, the Law of Segregation indeed is a general principle of genetics.

Monohybrid Crosses are Consistent!

Mendel’s Experiments – The Next Generation

• Mendel recognized that it is not always possible to tell what offspring will be like by inspecting the parent

• Mendel could test if tall plants were pure-breeds (homozygotes) or hybrid (heterozygotes) by the “back-cross” or “test-cross”

• What % would you predict for each genotype?

Tt

t t

Tt

t t

Mendel’s Hypothesis

Mendel’s Hypothesis• There are alternative forms for genes, the units that

determine inheritable characteristics (AA or Aa or aa)• For each inherited characteristic, an organism has two

alleles, one inherited from each parent. • A sperm or egg carries only one allele (A or a)for each

inherited characteristic, because allele pairs separate from each other during meiosis. At fertilization, the sperm and egg unite and restore the gene to the paired condition.

• When the two alleles of a pair are different, one is fully expressed (dominant) and the other is completely masked (recessive). The members of the pair may be identical (homozygous) or non-identical (heterozygous).

The alignment of one pair of homologs is independent of any other.

Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

Revisiting Meiosis

Incomplete Dominance

• Incomplete dominance is a blending of colors

• Dominance relationships may differ, but the Principle of Segregation is the same

Height is a polygenic trait

Polygenic Inheritance:When a Single Trait is Influenced by Many Genes

Multiple Alleles• Many genes are present

in three or more versions (alleles) – this is multiple alleles

• The human ABO blood group is determined by three alleles (IA, IB, and I) of a single gene

The AB phenotype (genotype IA

IB) is an example of codominance

Codominance

• The human ABO blood group also exhibits codominance – another genetic phenomenon

• Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote

Genetics of Blood Types

Phenotype Genotype Antigenon RBC

Antibodiesin Blood

DonationStatus

A A A or A itype A antigens

on surface of RBC

anti-B antibodies __

B BB or B itype B antigens

on surface of RBC

anti-A antibodies __

AB ABboth type A &

type B antigens on surface

of RBC

no antibodies universal recipient

O i ino antigens on surface

of RBC

anti-A & anti-B antibodies

universal donor

Sex-Linked Traits

XH Ymale / sperm

XH

Xhfe

mal

e / e

gg

s

XHXh

XH

Xh

XHY

Y

XH

XHXH XHY

XHXh XhY

2 normal parents,but mother is carrier

xXHY XHXh

Sex-Linked Traits

• Sex chromosomes have other genes on them, especially the X chromosome– Hemophilia in humans

• Blood doesn’t clot– Duchenne muscular dystrophy in humans

• Loss of muscle control– Red-green color blindness

• See green & red as shades of grey

Polydactyly • Individuals are born

with extra fingers and toes

• The allele for 6+ fingers and toes is dominant,while the allele for 5 digits is recessive

• Recessive is far more common! (1:500 have polydactyly)

Mendel performed dihybrid crosses to find out.

Are Different Characters Like Color and Shape Inherited Together or

Inherited Independently?

Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment.

Dihybrid Crosses