Transcript Ch. 11 - Introduction to Genetics

Introduction to Genetics
Chapter 11
Genetics
 The study of the inheritance of traits.
Gregor
Mendel’s Peas
 1800’s Austrian Monk
that detected patterns in
the inherited traits of pea
plants.
 Recognized as the “Father
of Genetics” – the study
of the inheritance of
traits.
The Work of Gregor Mendel
 Mendel observed several traits in his true-breeding peas.
 Mendel crossed pea plants containing contrasting traits.
 His results were surprising.
Principles of Dominance
Section 11-1
P Generation
Tall
F1 Generation
Short
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Principles of Dominance
Section 11-1
P Generation
Tall
F1 Generation
Short
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Principles of Dominance
Section 11-1
P Generation
Tall
F1 Generation
Short
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
The Work of Gregor Mendel
 Mendel decided “factors” that controlled traits were passed
on from parent to offspring, and that some of these factors
were dominant over others.
Genes and Alleles
 Genes – chemical factors on
chromosomes that control
traits.
 Alleles - different forms of a
gene often producing
contrasting traits.
Law of Dominance
 For a given trait, some alleles are dominant and others are
recessive.
 When inherited together, the dominant allele will always be
expressed over the recessive allele.
Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants
Section 11-1
Seed
Shape
Seed
Color
Seed Coat
Color
Pod
Shape
Pod
Color
Flower
Position
Round
Yellow
Gray
Smooth
Green
Axial
Tall
Short
Wrinkled
Green
White
Constricted
Yellow
Terminal
Round
Yellow
Gray
Smooth
Green
Axial
Plant
Height
Tall
Law of Segregation
 Alleles separate (segregate) from each other
during the formation of sex cells (gametes).
 During fertilization, alleles are brought back
together (recombination).
Probability
 What is the probability of getting tails when tossing a coin?
 What is the probability of getting tails after tossing the coin
four times and getting heads each time?
 Each coin toss is random and represents an independent
event. It has no influence over future tosses.
 Therefore, each toss always has a 1 in 2 (50%) chance of
getting tails.
Probability
 The segregation and recombination of alleles is random, so
probability can be used to predict the outcome of genetic
crosses.
 Probability is the likelihood that a particular event
will occur.
Punnett Squares
 Punnett squares use probability to predict the average results
of a genetic cross.
Terminology
 Genotype – the alleles for a trait. Symbolized using letters.
 Dominant traits are capitalized; recessive are lower case.
 T = tall
 t = short
 Phenotype – the expression of the trait. Descriptive.
 Tall, short, round, wrinkled…
Terminology
 Pure genotypes are called homozygous.
 TT, tt
 Hybrid genotypes are called heterozygous.
 Tt
 The phenotype of a pure or hybrid trait looks the same.
 tall
Punnett Squares
 Parent genotypes go on the outside of the Punnett square.
 Offspring genotypes go on the inside.
 Each square represents a 25% chance of offspring having a
particular genotype.
Punnett Squares
 For the following genetic crosses, use a Punnett square to
predict the phenotypes of the offspring.
Tt x Tt
2. Tt x tt
3. Yy x YY
4. RR x rr
1.
Independent Assortment
 Mendel followed the inheritance of several traits at a time.
 He concluded that factors (genes) were inherited
independently of each other. He called this “independent
assortment”.
 Ex: round peas could be yellow or green. Wrinkled peas could
be yellow or green. Texture and color were inherited
independently of each other.
Beyond Mendel……..
 Not all traits follow Mendel’s laws of dominance, segregation
and independent assortment.
Incomplete Dominance
 Some alleles are not completely dominant.
 Skin color and in flowers, petal color is determined by
incomplete dominance, and the resulting color is a blend.
Codominance
 Some alleles are codominant; both are fully expredded at the
same time.
 Human blood type and fur color is controlled by codominant
alleles.
Multiple Alleles
 Many genes have more than two alleles controlling the trait,
however, you can only inherit two of them: one from the
mother and one from the father.
 Human blood type and rabbit coat color.
Polygenic Traits
 Often a trait is controlled by more than one gene.
 Eye color in fruit flies involves the interactions of three
genes.
 Human skin color involves more than four genes.
Environmental Influences
 The expression of a trait is not determined solely by genetics.
 The environment can influence the expression of a gene.
 Skin color
 Plant height
Sexual Reproduction
 Human body cells have 46 chromosomes.
 During reproduction, an egg cell is fertilized by a sperm cell,
and their chromosomes unite.
 In producing sex cells, what must be done to the number of
chromosomes?
Meiosis
 Meiosis is the process by which sex cells are produced that
contain half the normal number of chromosomes.
Chromosome Number
 Homologous chromosomes contain genes for the same traits
(alleles).
 One is inherited from the mother and one from the father.
 Human body cells have 23 pairs of homologous chromosomes.
Chromosome Number
 Cells with pairs of homologous chromosomes are called
diploid (2N) cells.
 Body cells contain pairs of homologous chromosomes.
 In humans, 2N = 46
Chromosome Number
 Cells with only one chromosome from each homologous pair
are called haploid (1N) cells
 Sex cells (gametes) are haploid.
 In humans, 1N = 23
Meiosis
 Meiosis involves one replication of chromosomes and two
divisions of chromosomes.
 This is how the number gets cut in half.
Forming gametes
 In a male, the four halpoid cells become sperm.
 In a female, the four haploid cells become one egg and two or
three polar bodies.
Mitosis and meiosis comparison
mitosis
meiosis
One division
Two divisions
Produces two daughter cells
Produces four daughter cells
Maintains chromosome number
Reduces chromosome number by half
Daughter cells are diploid (2N)
Daughter cells are haploid (1N)
No genetic variation in daughter cells
Genetic variation in daughter cells
Used in asexual reproduction
Used in sexual reproduction
Linkage and Gene Maps
11-5
Independent Assortment
 Mendel observed that traits seemed to be inherited
independent of each other.
 We now know that it is entire chromosomes that sort
independent of each other.
Gene Linkage
 Genes located on the same chromosome tend to be inherited
together. They are “linked”.
 Ex: in fruit flies, red eyes and miniature wings
 Ex: in humans, blonde hair and blue eyes
Crossing-Over
 During meiosis when homologous chromosomes pair up
(synapsis) sister chromatids may cross-over each other, break
and rejoin resulting in new gene linkages.
 This is known as “crossing-over”.
Crossing-Over
Section 11-4
Crossing-Over
Section 11-4
Crossing-Over
Section 11-4
Crossing-Over
 Crossing-over during meiosis separates linked genes.
 The farther apart the linked genes, the greater the probability
they will be separated by crossing-over.
Gene Maps
 The rate at which linked genes are separated can be used to
create a gene map of a chromosome.
 The higher the recombination rate, the farther apart the two
genes.
 In other words, they will assort independently more often than genes
located close together.
Chromosome Maps
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gnd&
part=A272