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Mitosis, Meiosis, Genetic Variability, Sex Determination
Mitosis vs Meiosis
MITOSIS MEIOSISpreceded by replication of chromosomes?
# of rounds of cell division# of daughter cells# of chromosomes in daughter cells compared to parent cell
daughter cells genetically identical to parent cell?
happens in diploid cells, haploid cells, both, or neither?
crossing over (synapsis)?
Mitosis vs Meiosis
MITOSIS MEIOSISpreceded by replication of chromosomes?
yes yes
# of rounds of cell division 1 2# of daughter cells 2 4# of chromosomes in daughter cells compared to parent cell
same as parent cell
half of parent cell
daughter cells genetically identical to parent cell?
yes no
happens in diploid cells, haploid cells, both, or neither?
both(depending on organism)
diploid
crossing over (synapsis)? no yes
Genetic Variability
There are three mechanisms that contribute to genetic variation:
1. Independent assortment of chromosomes2. Crossing over3. Random fertilization
Independent Assortment
- Homologous pairs of chromosomes orient randomly during Metaphase I- Maternal and Paternal homologs assort into
daughter cells independently of other chromosomes- Each daughter cell gets at least one copy of the
chromosome
Independent Assortment
Possibility 1 Possibility 2
with n = 2there are
4 possibilitiesfor the lineupduring
Meiosis II
4 possible assortments of chromosomes in the gametes
Possibility 1 Possibility 2
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
2n Rule
2n = number of possible chromosome sorting combinations
- For humans (n=23), there are 223 = 8,388,608 possible combinations of chromosomes based on independent assortment alone!
Crossing Over
- During Metaphase I, homologous chromosomes pair up by gene and exchange homologous segments
Pair ofhomologs
Nonsisterchromatidsheld togetherduring synapsis
during Meiosis I(at anaphase I)
during Meiosis II(at anaphase II)
Daughtercells
Recombinant chromosomes
A single crossing over event leads to 4 genetically unique daughter cells!
Early inMeiosis I
Random Fertilization
- Each gamete has unique combination of genes - Fertilization between male and female
gametes occur randomly- Variation occurs due to different gene
combinations from male and female gametes
Question
If two identical twin females marry a set of identical twin males and have children and had a
DNA test on each others children would it say that Sister A’s kids belonged to Sister B because
of the identical DNA?
Article
Question
If two identical twin females marry a set of identical twin males and have children and had a
DNA test on each others children would it say that Sister A’s kids belonged to Sister B because
of the identical DNA?
Yes, but. . .
Based on the sensitivity of our modern paternity tests:
- All children would show up as if all children had the same two parents
- Genetically speaking, the children would be genetic siblings, not genetic cousins
However. . . .
“Identical” twins
Comparing 19 sets of adult identical twins:- Both contain the same set of genetic instructions
(genes on chromosomes)- DNA differs at various points on their genome
- Copy number variants: Twins had a different number of copies of the same gene
- Mutations occur during early stages of zygote development
- “…the genome you’re born with, is not the genome that you die with…”
Normal Human Chromosomes
Homologous Chromosomes
- Set of one maternal chromosome and one paternal chromosome that pair up with each other during meiosis
- These copies have the same genes in the same locations, or loci
- Humans have 22 pairs of homologous chromosomes
But what about the last pair?!
- 23rd pair of chromosomes are not always homologous
- This last pair of chromosomes have two choices:- XX – homologous- XY – not homologous
- Along with other characteristics, these chromosomes are responsible for genes that determine the sex of a mammal
Sex Determination in Humans
- XX = female- XY = male
- Only males can posses a Y chromosome- A Y chromosome can only be received from your
father
- After fertilization, sex if determined by the genetic composition of the sperm (if X or Y is present)
What does the Y chromosome do?
- 0 to 6 weeks of embryonic development- Embryo develops as female
- 7th week of embryonic development- Y chromosome triggers release of androgens that
stimulate development of male reproductive organs
- If no androgens are released, embryo continues to develop as female due to release of estrogens
Male Parent x Female ParentXY x XX
Sex-Linked Inheritance
- Sex chromosomes contain genes for “sex-linked” traits
- Sex linkage- Often sex linked traits are carried on the X
chromosome- Sex-linked genes are expressed more often in
males than females- Males carry 1 copy, Females carry 2 copies of gene
Sex-linked Notation
If we assume B is dominant over b:XBXB = “normal” femaleXBXb = carrier femaleXbXb = affected female
XBY = “normal” maleXbY = affected male
Sex-linked Genes
- Examples in Humans:- Colour blindness- Haemophilia- Duchenne muscular dystrophy (DMD)
- Eye colour in fruit flies is the most famous example of a sex-linked trait
Fruit FliesDrosophila melanogaster
Observing Fruit Flies
Trait PhenotypeEye colour Red eye (wild type)
White eye (mutant)P Phenotypes
Wild type (red-eyed) female x White-eyed male
F1 Phenotypes All red-eyed
Conclude red eye is dominant to white eye
Hypothesis
F2 Phenotypes Red eye White eye
Numbers 347082%
78218%
A cross between the F1 flies should give us: 3 red eye : 1 white eye
An interesting observation…
F2 Phenotypes Red-eyed males
Red-eyed
females
White-eyed males
White-eyed
females
Numbers 1011 2459 782 0
24% 58% 18% 0%
A reciprocal cross
Morgan tried the cross the other way roundwhite-eyed female x red-eyed male
Result- All red-eyed females and all white-eyed males
This confirmed what Morgan suspected.The gene for eye colour is linked to the X chromosome.
Genetic diagram for sex linked genes
Character Trait Alleles
Eye colour Red eye R
White eye r
Genotypes Phenotypes
XRXR
XRXr
XrXr
XRYXrY
Genetic diagram for sex linked genes
Character Trait Alleles
Eye colour Red eye R
White eye r
Genotypes Phenotypes
XRXR
XRXr
XrXr
Red-eyed female Red-eyed female
White-eyed female XRYXrY
Red-eyed male White-eyed male
Let’s go back to our original cross:
P Phenotype Wild type (red-eyed)
female
x White-eyed male
Genotype XRXR XrY
Gametes XR XR Xr Y
Xr Y
XR XRXr XRY
XR XRXr XRY
F1 x F1
F1 Phenotype Red-eyed female
x Red-eyed male
Genotype XRXr XRY
Gametes XR Xr XR Y
XR Y
XR XRXR XRY
Xr XRXr XrY
F2 Phenotype Females Males
Red-eyed
White-eyed
Red-eyed
White-eyed
Expected All None 50% 50%
Observed 2459 0 1011 782
This gene has its LOCUS on the X-chromosome
It is said to be SEX-LINKED
Sex linked inheritance: Dominant
• When the dominant gene is on the X chromosome:– Affected males pass to all of their daughters and
none of their sons • Genotype= XAY
• If the mother has an X- linked dominant trait and is homozygous (XAXA), all of her children will be affected
• If the mother is heterozygous (XAXa), there is a 50% chance of each child being affected
Sex linked dominant problem
The barred pattern of chicken feathers is inherited by a pair of sex linked genes, B for barred, b for no bars. If a non-barred female is mated to a barred male,
Genotypes: a) What will the genotype and phenotype of the
offspring be?b) What will be the genotype and phenotype of the
offspring produced by mating an F1 male with an F1 female?
Sex linked Inheritance: Recessive
• When you are following the recessive trait located on the X chromosome
• More males than females affected (males inherit X from mother)
• Females can only inherit if the father is affected and mother is a carrier (hetero) or affected (homo)
• An affected female will pass the trait to all her sons– Daughters will be carriers if father is not affected
• Males cannot be carriers (only have 1 X so either affected or not)
• Can skip generations• E.g. colour blindness, hemophilia, Duchene muscular
dystrophy
Sex linked recessive problem
Red-green colour blindness in men is caused by the presence of a sex-linked recessive gene b, whose normal allele is B.a) Can two colour blind parents produce a normal son?b)Can they produce a normal daughter?c) Can two normal parents produce a colour blind son or
daughter?d)Can a normal daughter have a colour blind father or
mother?e) Can a colour blind daughter have a normal father or mother?
