Download 3.4 Mendel

Introduction to Mendelian Genetics
Gregor Mendel (1822-1884)
• Used pea plants to study
genetics, heredity and
variation.
• Looked at 7 different traits
that only had two variations
(two different alleles)
• Discovered Mendel’s Laws of
genetics
19th Century Britain
• Before Mendel, from breeding ornamental
plants, scientists believed in a blending
theory of inheritance.
• Charles Darwin theorized that offspring
have variations of their parents'
characteristics, but he was unable to
explain why (1870s).
Gregor Mendel (1822-1884)
• Austrian monk
• Considered the father of
genetics (now aka Mendelian
Genetics)
• Why the pea plant?
– grows easily and quickly
– has seven traits that are easily
identified
– it is easy to manipulate
7 traits Mendel looked at
Remember:
• We have 2 copies of every gene (2 alleles)
– We get 1 allele from father, 1 allele from
mother
– Both of these alleles combined give us our
traits
• A trait is true bred if both of the alleles are
the same
– For example, a true bred “tall” plant would
have 2 “tall” alleles
Mendel's Pea Plant Experiment
• Mendel selected true bred plants for different traits and cross-fertilized
them to see what would happen
• For example:
– He bred true bred TALL with true bred SHORT
• What do you think the offspring looked like?
X
• Normally, you would assume that
they would be medium height
(blended between tall and short)
X
• However, Mendel found that 100%
of the offspring plants were tall!
• This shows that some alleles (like
tallness) are dominant over other
alleles
• Mendel then let the F1 generation self-pollinate (fertilize
each other)
• F1 generation – (filial generation) offspring from the cross
(reproduction) of the Parent or P generation.
x
• this time most were tall, but some were short and they were
ALWAYS 3:1 ratio
• This shows that even though “tall” dominated “short”, the
short allele is still hidden in the organism
Why?
• Each plant in the F1 generation carried an allele from the P
generation; a tall allele and a short allele.
– This is a hybrid.
– AKA heterozygous
• Traits are dominant or recessive. If a dominant trait is
inherited, it will be expressed.
Mendel’s “Key Terms”
• Phenotype: the appearance of traits in an organism (eg.
short or tall)
• Genotype: the specific alleles that an organism has (ex.
Tt or TT or tt)
• Every individual has a phenotype and genotype for every
trait.
Genotype vs. Phenotype
Which Allele is expressed?
Can you tell the genotype of an
individual by their phenotype?
A flower species is dominant for the purple trait
and recessive for the white trait. What would
happen if we crossed two hybrid (heterozygous)
parents? We use a Punnett square to find out.
Punnett Square
Genotypic proportions
The genotypic proportions of the offspring
from the cross of two heterozygous
parents (Bb) are:
• BB __/4
• Bb __/4
• bb __/4
Phenotypic proportions
The phenotypic proportions of the offspring
are:
• Purple __/4 or __%
• White __/4 or __%
Monohybrid cross problem
Curly hair is dominant over straight hair. If a
mother is homozygous dominant for curly
hair and the father has straight hair, what
type of hair will the children have?
Test Crosses
• When geneticists want to know if an individual is
heterozygous or homozygous
– Why? For breeding purposes
• Cross between unknown phenotype and homozygous
recessive individual
– (Pp or PP) x pp
• Pp x pp = 50% purple, 50% white
• PP x pp = 100% purple
Mendel's Laws and Principles
• Law of Segregation: when any individual produces
gametes, the copies of a gene separate so that
each gamete receives only one copy of a gene and
therefore only one allele for all possible traits.
Principle of Segregation Demystified
Law of Segregation
Segregation
The principle of segregation is explained by the behavior of homologous
chromosomes at meiosis.
• Law of Independent Assortment: alleles of different
genes assort independently of one another during
gamete formation, so different traits are inherited
independently of each other