Mitosis, Meiosis, Genetic Variability, Sex Determination

Mitosis vs Meiosis MITOSISMEIOSIS preceded 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 MITOSISMEIOSIS preceded by replication of chromosomes? yes # of rounds of cell division12 # of daughter cells24 # of chromosomes in daughter cells compared to parent cell same as parent cell half of parent cell daughter cells genetically identical to parent cell? yesno happens in diploid cells, haploid cells, both, or neither? both (depending on organism) diploid crossing over (synapsis)?noyes

Genetic Variability There are three mechanisms that contribute to genetic variation: 1.Independent assortment of chromosomes 2.Crossing over 3.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 = 2 there are 4 possibilities for the lineup during Meiosis II  4 possible assortments of chromosomes in the gametes

Possibility 1 Possibility 2 Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4

2 n Rule 2 n = number of possible chromosome sorting combinations  For humans (n=23), there are 2 23 = 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 of homologs Nonsister chromatids held together during synapsis during Meiosis I (at anaphase I) during Meiosis II (at anaphase II) Daughter cells Recombinant chromosomes A single crossing over event leads to 4 genetically unique daughter cells! Early in Meiosis 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?!  23 rd 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  7 th 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 Parent XY 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: X B X B = “normal” female X B X b = carrier female X b X b = affected female X B Y = “normal” male X b Y = 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 Flies Drosophila melanogaster

Observing Fruit Flies TraitPhenotype Eye colourRed eye (wild type) White eye (mutant) P Phenotypes Wild type (red-eyed) female x White-eyed male F 1 Phenotypes All red-eyed Conclude red eye is dominant to white eye

Hypothesis F2F2 PhenotypesRed eyeWhite eye Numbers % % A cross between the F 1 flies should give us: 3 red eye : 1 white eye

An interesting observation… F2F2 PhenotypesRed- eyed males Red- eyed females White- eyed males White- eyed females Numbers %58%18% 0%

A reciprocal cross Morgan tried the cross the other way round white-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 CharacterTraitAlleles Eye colourRed eyeR White eyer GenotypesPhenotypes XRXRXRXrXrXrXRXRXRXrXrXr XRYXrYXRYXrY

Genetic diagram for sex linked genes CharacterTraitAlleles Eye colourRed eyeR White eyer GenotypesPhenotypes XRXRXRXrXrXrXRXRXRXrXrXr Red-eyed female White-eyed female XRYXrYXRYXrY Red-eyed male White-eyed male

Let’s go back to our original cross: PPhenotypeWild type (red-eyed) female xWhite-eyed male GenotypeXRXRXRXR XrYXrY GametesXRXR XRXR XrXr Y XrXr Y XRXR XRXrXRXr XRYXRY XRXR XRXrXRXr XRYXRY

F 1 x F 1 F1F1 PhenotypeRed-eyed female xRed-eyed male GenotypeXRXrXRXr XRYXRY GametesXRXR XrXr XRXR Y XRXR Y XRXR XRXRXRXR XRYXRY XrXr XRXrXRXr XrYXrY

F2F2 PhenotypeFemalesMales Red- eyed White- eyed Red- eyed White- eyed ExpectedAllNone50% Observed 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= X A Y If the mother has an X- linked dominant trait and is homozygous (X A X A ), all of her children will be affected If the mother is heterozygous (X A X a ), 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 F 1 male with an F 1 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?