Download Genetics

Introduction
• Genetics: the study of the transfer of traits and
hereditary material from parents to offspring.
• Asexual reproduction: The offspring is exactly
copy of the parent. Only one parent is involved.
• Sexual reproduction: Two parents are involved.
-Genes from 2 organisms (parents) of the same
species get mixed.
sex cells = gametes = sperm and ova
• What is the purpose of sexual reproduction?
– Greater variety
• Why are animals diploid (2 copies of each
chromosome)?
– Most of “genetics” is based on the fact that
animals have 2 copies of each gene.
• What is the purpose of meiosis:
– To keep the chromosome number constant
•
• Diploid (2n): having 2 sets of chromosomes
• Haploid (1n): having 1 set of chromosomes
• Homologous chromosomes have the same
genes.
Meiosis
• Meiosis is cell division which produces gametes.
• Meiosis cuts the chromosome number in half.
-Example: a human body cell has 46 chromosomes
and a human sex cell has 23 chromosomes
• Meiosis occurs after DNA replication, and contains
two cell divisions ( Meiosis I and Meiosis II), with no
Interphase between them.
Meiosis l – homologous chromosomes separate
This results in 2 daughter cells that are NOT the same
(DON’T NEED TO KNOW ALL DETAILS)
Meiosis ll –
similar to mitosis: sister chromatids separate
(DON’T NEED TO KNOW ALL DETAILS)
Crossing over provides more genetic variation
Occurs at prophase of Meiosis I
Genetics
Gregor Mendel observed pea plants
with different traits ( flower color,
plant size, pea shape..)
Self-pollination
• Most plants produce both sperm and ova
• Pea plants can self-pollinate.
• Self pollination leads eventually to pure –bred
plants.
• Pure-bred plants produce offspring exactly like
themselves (if self-pollinated).
• Mendel crossed various pure-bred plants.
Life Cycle of the Pea Plant
Cross-Pollinated Plants
Dominant and Recessive Traits
• A dominant trait overshadows a recessive
trait.
• Mendel crossed yellow seeds x green seeds.
All offspring had yellow seeds. He did not get
plants with green seeds.
• P – parental generation
• F1- first filial generation (from 2 purebred
parents)
• F2- second filial generation (F1 X F1)
Mendel then crossed the F1 offspring
among themselves. In the F2, he got 3/4
yellow seeded plants and ¼ green seeded
plants.
Mendel’s conclusions
• There are 2 alternative forms of a gene, one
for green seeds and one for yellow seeds.
These 2 forms are called alleles.
• Peas have 2 alleles of each gene.
• Yellow is dominant and green is recessive.
Modern Genetics
• Genes are segments of DNA on a chromosome
that make a particular protein that is
responsible for a particular trait (seed color,
skin color, eye color, type of hair, etc.)
• Homozygous is an organism with two identical
alleles of a gene for a given character:
- YY (yellow) or yy (green).
• Heterozygous is an organism that has different
alleles for a character: Yy. Only the dominant
gene will be expressed.
• Phenotype is an expressed, observable trait.
– Example: color of the seed, yellow phenotype and
green phenotype.
• Genotype is the combination of specific alleles
in the offspring.
- Example: YY and Yy genotypes for yellow color, and
yy genotype for green color.
The Punnett Square
• It demonstrates simple genetic
possibilities and probabilities in chart
form.
• Parent’s alleles are placed on both
sides of the chart
The Punnett Square
F2 Phenotype and genotype ratio
Phenotype ratio in a Dihybrid Cross
(DON’T NEED TO KNOW DETAILS)
The Law of Independent Assortment
• The traits are not stuck together. They sort
independently.
Probabilities
•
•
1.
•
Rule: probabilities of events that must happen at the same time are multiplied
Coin: H-head h-tails
Coin # 1 H probability ½ ( 50%)
Coin #2 H probability ½ (50%)
Probability for HH: ½ x ½ = ¼ ( 25%)
2. Probability for hh: (25%)
3. Probability for Hh: (25%)
4. Probability for hH: (25%)
5. Hh and hH are basically the same. To calculate the probability of one head
and one tail, add the two separate probabilities.
• ¼ + ¼ = 2/4 (50%)
• In a 4-person card game in which all the cards
are dealt out, what is your chance of getting
the king of spades?
• What is your chance of getting the king and
ace of spades?
• What is your chance of getting the queen, king
and ace of spades?
Pedigree ChartGenetic History of a Person Carrying a Mutation
• Using eye color (blue recessive or brown
dominant), what conditions would prove that
a man is not the father of a child?
Answer
• If child has brown eyes, and both man and
mother have blue eyes.
Polygenetic Inheritance
• More then one gene determines a particular
trait. Examples: wheat grain color, skin color,
body height.
• Such a trait exist in a range (it does not come
in just two variations).
Sex Determination
• X and Y chromosomes are a pair, but they do
not carry the same genes. The Y chromosome
is much smaller.
• XY - male individual
• XX - female individual
• So the Y chromosome makes a person a male
• Females have 2 X chromosomes, but males
have only 1.
Genes linked to the X sex chromosome
• Sex chromosomes also carry genes for other
traits. The X chromosome carries many more
genes than the Y chromosome.
• Since a male has only one X chromosome, all
genes on that chromosome will be expressed.
• A bad gene on the X chromosome is much
more likely to be expressed in a male than in a
female.
• My father was color blind (X-linked recessive).
1. I am not, but do I have to worry that my
children will get my father’s color blind gene
from me?
2. If I had a sister, would she have to worry
about this?