Download Meiosis and Genetics

9/6/2015
Make a structural concept map of
the cell cycle
Meiosis and Genetics
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Humans have ____ chromosomes in each cell
This is a human karyotype
(a technique that organizes chromosomes by type and size)
What pattern do you notice?
For each type of chromosome there are _____
Gametes are produced by a different type of cell division
called meiosis
# of cells produced
# of chromosomes in daughter
cells vs. parent cell
In what part of the body does
meiosis take place?
Original cell is ________
Gametes are________
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Accidents during meiosis can alter
chromosome number in gametes
Cause: Non disjunction
the chromosome fails to move apart properly
Result: gametes have an extra or a one less chromosome
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Trisomy: inheriting 3 copies of a chromosome
Most common trisomies in
newborn children are
13, 18 and 21
Screening and Diagnosing
Triple Screen Test
Amnio and CVS
The connection between genes and traits
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Genes determine traits
• Genotype is the genetic make up of an organism (gene)
• Phenotype is the appearance of an organism (trait)
Genotype determines Phenotype
Closer look at Genotype:
the combination of alleles an individual carries
• Chromosomes contain different genes
Gene for
color of eyes
• Gene codes for a protein (trait)
• Alleles are versions of a gene
• You are a diploid organism
You carry two chromosomes of each type
You carry two of each type of gene
Homozygous individual: alleles are the same
Heterozygous individual: alleles are different
What is the phenotype of these individuals?
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•
•
Homozygous genotype  phenotype reflects genotype
Heterozygous genotype  one of the alleles is fully expressed
The allele that is expressed
is the dominant allele
The allele that does not show up
is the recessive allele
•
Three genotypes possible:
Homozygous recessive, homozygous dominant or heterozygous
•
Two phenotypes possible: dominant or recessive
Organism’s phenotype does not always reveal its genotype
Symbols used in genetics
• We assign a letter to each gene
• If gene has two alleles we use upper and lower case
where Upper case is the dominant allele
Lower case is the recessive allele
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Part 1. Determine your partner’s phenotype and possible genotype
Traits
Earlobes: detached is dominant over attached
E
e
Tongue: tongue rolling is dominant over not being able to roll tongue
T
t
Hair line: widow’s peak is dominant over straight hairline
W
w
How can two individuals with different genotypes
for a particular trait
be identical in phenotype?
How are these traits (alleles) inherited?
What happens to alleles during meiosis?
What happens to alleles after fertilization?
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What happens to the alleles during meiosis?
•
During meiosis,
If we look at one gene only:
A gamete has only one allele
•
How many types of gametes does
each genotype produce?
•
Homozygous individual produces only
one kind of gametes
•
Heterozygous individual produces two
kinds of gametes
Genetics in Class
Practice # 2
In peas, the allele for purple flowers (P) is dominant
to the allele for white flowers (p).
What gametes would be produced by
a plant that has white flowers?
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What happens during fertilization?
Fertilization creates allele pairs again
Gamete types
Parent 1:
Genotype Aa
During fertilization
A
Offspring:
Genotype aa
a
Parent 2:
Genotype Aa
in the offspring
A
Chance determines which
alleles
the offspring receives from
each parent
a
We need to consider
all possible combinations
and
estimate probabilities
for each offspring phenotype and genotype
Parent 2: Aa Gametes
from parent 2
Parent 1: Aa
Gametes from parent
1
Probabilities of offspring
being:
AA:
Aa:
aa:
Dominant phenotype:
Recessive phenotype:
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Punnett square shows all the possible
offsprings from a matting
STEPS TO SUCCESS:
1. determine the alleles and
genotypes of the parents
X
2. write down your "cross" (mating)
3. draw a p-square
4. "split" the letters of the genotype for each
parent & put them "outside" the p-square
5. determine the possible genotypes of the
offspring by filling in the p-square
6. estimate probabilities for
genotypes & phenotypes of offspring
7. bask in the glow of your accomplishment!
Practice # 3
Assume tall (T) is dominant to dwarf (t).
If a homozygous dominant individual is crossed with a
dwarf.
What will the offspring look like?
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Fill in the blanks:
allele, recessive, homozygous, dominant, chromosomes
b)
Practice # 4
Pea plants are tall if they have the genotype TT or Tt,
and they are short if they have genotype tt.
A tall plant is mated with a short plant.
Half the offspring are tall, and half are short.
What do you know about the tall plant?
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Complete dominance is rare
One gene  one trait
COMPLETE DOMINANT GENES
CODOMINANT GENES
INCOMPLETE DOMINANT GENES
MULTIPLE ALLELES
SEX-LINKED GENES (genes found in the X or Y chromosomes)
One gene  many traits
PLEIOTROPY
Many genes  one trait
POLYGENIC INHERITANCE
A gene with Incomplete Dominance results in
in the heterozygous individual expressing
an intermediate phenotype
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A gene with Codominance results in
in a heterozygous individual expressing
both recessive AND dominant phenotypes
EXAMPLE: gene for color of hair
AOB blood type
AOB blood gene codes for
the type of antigen found
in the membrane of red blood cells
AOB blood gene is a codominant gene
AOB blood gene has more than two alleles (multiple alleles) Alleles: A, B, O
Genotypes
Phenotypes
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Practice # 5
A man who has type B blood
And
a woman who has type A blood
have a child with type O blood.
How is this possible?
A single characteristic may be influenced by many genes
Polygenic inheritance
Additive effect of 2 or more
genes on a single trait
Examples of polygenic traits:
Height, skin or eye color
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Sex chromosomes determine sex in many species
In some cultures,
The mother is blamed for not bearing a boy,
is this justified?
Each chromosome has unique
Genes that are not found in the other
Types of sex determination:
In humans and fruit flies have the X-Y system
(XX = female, XY = male),
In some insects sex is determined by the X-O
system
in which females have two copies of the X
chromosome and males only one.
Therefore, the sex of the offspring will be
determined by whether or not an X chromosome is
present in the sperm that fertilizes the egg.
The Z-W system occurs in some birds, insects, and
fish.
In this system the chromosomes found in the egg
are variable
Here, it is the female (not the male) that
determines the sex of the offspring.
Bees and ants display a fourth type of sex
determination, the haplo-diploid system .
Females develop from fertilized eggs and are
diploid, whereas males develop from unfertilized
eggs and are haploid.
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Genes carried in the sex chromosomes
exhibit a unique pattern of inheritance
We will focus on X linked genes
Example: Hemophilia is a
X linked recessive disease
Possible alleles
Solving genetic problems for X linked genes
Problem should state that
it is a sex linked gene
Allele is attached to the X chromosome
Need to determine gender of offspring
In order to determine phenotype
Color blindness is a recessive trait
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Practice # 6
A couple are both normal,
but their son suffers from hemophilia.
What fraction of the couple’s children
are likely to suffer from hemophilia?
What fraction are likely to be carries?
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