Download Chapter 11 Introduction to Genetics

The Work of Gregor Mendel
What is genetics? Why study
it?
Genetics
= study of heredity
Heredity = passing on of
characteristics from parents to
offspring
Geneticists study genes, stretches of
DNA, that code for proteins
What are some features
in humans that everyone
has in common?
What are some
features that are
different from human
to human?
Why pea plants?
 Easy
to grow and mature quickly
 Structure & method of pollination made
them easy to use in controlled experiments:
could self-fertilize or cross-fertilize
 Different plants show contrasting traits
 Characteristic
= heritable feature that varies
among individuals, ex: flower color
 Trait = different varieties for a characteristic, ex:
purple or white flowers
Why did Mendel Choose the Pea?

Flower Structure/ allowed controlled mating

Many variable traits/ simple traits

Fast Generation Time
Gregor Mendel
 Genetics
– the scientific
study of heredity
 Gregor
Mendel – (born in
1822) worked with true
breeding garden peas to
study inheritance
 True
breeding – produce
offspring identical to
themselves when they selfpollinate
White
1
Removed
stamens
from purple
flower
Flower color
Stamens
Carpel
PARENTS
(P)
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Flower position
2 Transferred
Purple
pollen from
stamens of white
flower to carpel
of purple flower
3 Pollinated carpel
matured into pod
4
Planted
seeds
from pod
OFFSPRING
(F1)
Stem length
Genes and Dominance
 Mendel
crossed plants with seven different pea
plant traits that each had two contrasting
characteristics and studied the offspring.
P
generation – original pair of plants
 F1 generation – 1st generation of offspring
 F2 generation – 2nd generation of offspring
Mendel’s Crosses
Self-pollination
– sperm cells fertilize egg
cells of the same plant (i.e. one parent,
but still sexual reproduction)
True-breeding plants – if allowed to selfpollinate, they would always produce
offspring identical to themselves
 Ex:
true-breeding short plants always have offspring
that are short when then self-pollinate
The F1 Cross
 Mendel
wondered if the recessive alleles had
disappeared or if they still existed in the F1
generation.
 He let the F1 plants self-pollinate creating the F2
generation.
 Traits
controlled by the recessive alleles reappeared
in 1/4th of the F2 generation.
Mendel made two conclusions:
1.
Inheritance is determined by factors passed on
from one generation to the next.
•
•
2.
Genes – pieces of DNA that determine a trait
Alleles – different forms of genes
Some alleles are dominant and others are
recessive. (Principle of Dominance)
•
•
Dominant allele – always expressed as a trait
when present (represented by a capital
letter)
Recessive allele – only expressed when the
dominant allele is not present (represented
by a lower case letter)
Current genetic terminology
Phenotype
is the outward
appearance; ex: tall or short height
Memory trick:
 Genotype
is the genes
 Phenotype is the physical feature you see
General Terms:
 Homozygous
– having two identical alleles for a trait
(ex: TT – homozygous dominant or tt – homozygous
recessive)
 Heterozygous
trait (ex: Tt)
 Hybrid
– having two different alleles for a
– offspring of crosses with different traits
 Genotype
TT, Tt or tt)
– genetic makeup of an individual (ex:
 Phenotype
– physical appearance of an individual
 Probability
– the likelihood that an event will occur
(ex: brown, tall)
The Albino Trait
Genetics and Probability
 Mendel
realized that probability could be used to
explain the results of genetic crosses.
Punnett Squares:
T – tall
t – short
 Possible
25% TT – tall
50% Tt – tall
25% tt – short
offspring of a Tt x Tt cross:
75% chance - tall and 25% - short (3:1 ratio)
Mendel’s Law of Segregation:
As seen in P and F1 generations
Dad
T = tall (dominant)
t = short (recessive)
Parental!
TT x
Mom
tt
meiosis
Gametes?
Punnett Square
for monohybrid
crosses
Mendel’s Law of Segregation:
As seen in P and F1 generations
Dad
T = tall (dominant)
t = short (recessive)
Parent 2 Gametes
Parent 1 Gametes
T
T
t
Tt
tall
Tt
tall
t
Tt
tall
Tt
tall
Mom
TT x
Parental!
tt
meiosis
T
T t
t
Gametes
Genotype = 100% Tt
Phenotype = 100% tall
Mendel’s Law of Segregation:
As seen in P and F1 generations
Dad
Mom
Tt
T = tall (dominant)
x Tt
t = short (recessive)
T
t
T
TT
tall
Tt
tall
t
Tt
tall
tt
Probabilities?
T
t
T
t
Offspring = F2
generation
genotype = 1 TT: 2 Tt: 1 tt
short phenotype = 3 tall : 1 short
Practicing with Punnett Squares
Parents – Tt and tt
T
t
Parents – TT and Tt
T
T
t
Tt
tt
T
TT
TT
t
Tt
tt
t
Tt
Tt
Offspring:
50% Tt – tall
50% tt – short
ratio - (2:2) or (1:1)
Offspring:
100% TT or Tt – tall
ratio - (4:0)
Beyond Dominant & Recessive
Some genes are neither dominant nor recessive
and many traits are controlled by multiple alleles
or multiple genes.
 Incomplete
Dominance – neither allele is
completely dominant so the heterozygous
phenotype is a blending of traits
 Ex:
four o’clocks - red flower crossed
with white flower produces pink flower
Codominance
Q: What does “cooperate” mean?
A: Operate together
 Codominance = when both alleles are expressed;
neither is dominant nor recessive
Q: If the two homozygotes are red and white, what’s
the phenotype of the heterozygote?
A: The heterozygote shows red and white hairs
 Roan
coat in cows and horses
 Both
red and white hairs are present
 Codominance
– both alleles are dominant
so both traits show in the heterozygous
phenotype

Ex: chickens – white chicken crossed with
black chicken produces black and white
speckled chicken
 Multiple
Alleles – many genes have three
or more alleles of the same gene

Ex: blood type
 Polygenic
traits – traits that are controlled
by two or more genes; show a wide range
of phenotypes.

Ex: skin color is controlled by at least 4 genes
Sex Linked Traits
Q: What are the two types of chromosomes?
A: Sex chromosomes and autosomes
Q: How many sex chromosomes are there?
What are they? What do the different
combinations mean?
A: Two sex chromosomes – X & Y
XX = female
XY = male
Color-blindness test!
Color-blindness test!