Download Introduction to Genetics

Introduction to Genetics
Chapter 11
Genetics:
• The scientific study of heredity
• The study of how gametes (from
meiosis) combine and which traits are
expressed!
History
• Gregor Mendel- Austrian
monk, teacher, and gardener
• Studied heredity of pea plants
– Which traits (color, texture, etc.)
were expressed under different
conditions and breeding
situations
• Drew two conclusions from his
experiments
Conclusion 1
• Biological inheritance is determined by
factors that are passed down from one
generation to the next.
– These “factors” are called genes.
– Every trait is controlled by one or more genes.
• Different forms of genes are called alleles.
• Example: eye color=trait
blue, brown, green, hazel=alleles
• Segregation is the separation of alleles during
gamete formation---during meiosis!!
Conclusion 2
• Some alleles are dominant, some are
recessive.
– If an organism has a dominant allele for a
trait, that is the allele that will be expressed.
– Example: Yellow peas are dominant over
green peas. If the yellow pea allele is present
even once, the peas will be yellow.
How did Mendel do it?
• Fertilization is the meeting of the gametes (in
sexual reproduction).
• True-breeding plants self-pollinate to produce
exact copies of themselves (clones).
• Cross-pollination occurs when gametes from two
different individuals are combined.
– Cross-pollination of individuals with different traits
makes a hybrid.
Mendel crossed pea plants to determine what traits
would be expressed in the offspring.
How Pollination works:
Examples of
Segregation:
Aa
A
AA
x
Aa
a
A
Aa
Aa
A
a
aa
a
A
AA Aa
a
Aa aa
• The first diagram shows
segregation: each parent’s alleles
separate and combine with the
alleles of the other parent.
• The second diagram is a Punnett
Square. It is used to show the
segregation of alleles and the
probability of combinations.
• What words can we use to
describe this cross?
Description:
• Parents (Aa) are heterozygous: having two
different alleles.
• There is a 50% chance the offspring will
also be heterozygous.
• There is a 25% chance the offspring will be
homozygous and recessive. (aa)
• There is a 25% chance the offspring will
have AA, which is homozygous and
A
dominant. (AA)
– Homozygous: having two of the same
a
alleles.
A
a
AA Aa
Aa aa
Phenotype and Genotype
• Phenotype is a word used to described the
physical characteristics expressed by the
genes. (Think pheno=physical!)
• Genotype is a word used to describe the
genetic makeup of that characteristic.
(Think geno=genes!)
– Example 1: “TT” would be the genotype while
“tall” would be the phenotype.
– Example 2: “Tt” would be the genotype while
“tall” would be the phenotype.
Question:
If A represents normal color and a
represents albino:
1.Which is dominant…normal or albino?
2.What are the phenotypes of both parents?
3.What are the phenotypes of the four
possible offspring?
A
a
A
AA Aa
a
Aa aa