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Mendel & heredity
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• Heredity- transmission of traits
from parent to offspring.
• Genetics- the study of heredity
• Gregor Mendel
• The “father” of genetics
• Austrian monk
• Studied the garden pea to develop
a set of rules to predict the
patterns of heredity.
• Mendel used the garden pea
because:
• It grows quickly
• Has many varieties
• Able to self-pollinate or true breed
Mendel’s experiments
• 1. produced a true-breeding P
generation (parent)
• 2. produced the next generation, F1
(filial) generation, by cross pollinating
plants.
• 3. produced 3rd generation, F2 (filial)
generation by self- pollination
• See picture
Mendel’s Theories
• Mendel thought that certain
“factors” were passing on
traits.
• We now know the factors are
genes.
• An organism has 2 genes for
each trait.
• One gene came from the female
gamete and one from the male
gamete.
Meiosis
Simple patterns of Heredity
aka principle of dominance
• Dominant gene- the gene
that can cover up a
recessive gene
• Recessive gene- the gene
that can be covered up.
• If you have one dominant gene
and one recessive gene you will
have the dominant trait.
• If you have two dominant genes
you will have the dominant trait.
• If you have two recessive genes
you will have the recessive trait.
Examples in peas
Cross
Result
Tall X short
Tall
Green pea X
Yellow pea
Smooth pea X
wrinkled pea
Yellow pea
Smooth pea
• Use Upper case letter for
dominant gene
• ex- T is for tall pea plant
• Use lower case letter for
recessive gene
• ex- t is for short pea plant
• Homozygous- if both genes for the
trait are the same ex-TT
• Heterozygous- if the genes for the
trait are different ex- Tt
• Allele- one of the copies of a gene
ex- T
• Phenotype- the visible trait of the
organism. Ex- Tall
• Genotype- the combination of genes
for a trait. Ex- TT, Tt, or tt
Mendel’s Laws
• 1. The Law of Segregation: Two
alleles for a trait separate during the
formation of gametes.
• 2. The Law of Independent
Assortment: alleles for different traits
separate independently of one another
during formation of gametes if they are
on different chromosomes.
• ex- the gene for eye color separates
independently of the gene for hair color.
• In humans, 6 fingers on one hand is dominant and
5 fingers is recessive.
• 1. What is the genotype of an individual that is
homozygous dominant?
• 2. What is the phenotype of an individual that is
homozygous dominant?
• 3. What is the genotype of an individual that is
heterozygous?
• 4. What is the phenotype of an individual that is
heterozygous?
• 5. What is the genotype of an individual with 5
fingers?
Complex Patterns of Inheritance
• 1. Incomplete dominance- a
heterozygous individual show a trait
that is a blend of the 2 genes.
• Ex- snapdragons- if flower has a
dominant red allele and a recessive
white allele the flower is pink.
• 2. Codominance- 2 different
dominant alleles can be expressed at
the same time in heterozygous
individuals.
• Ex- Roan color in horses. Both red
and white hair color genes are
dominant. The horse has red hairs
and white hairs.
• 3. Multiple alleles- some traits
have more than 2 alleles that
determine it.
• Ex- 3 alleles determine blood type
A, B, and O
Polygenic Inheritance
• More than one gene effects the
trait.
Ex- height, weight, skin color
Only about 600 traits in humans
are determined only from one
gene.