Genetics Lectures 11-13 Mendelian Genetics + Meiosis + Linkage Flashcards
what was mendels first law
law of segregation: when gametes are forming, the paired hereditary factors segregate equally
whats mendels second law
during gamete formation, the paired hereditary factors segregate independently: independent assortment genes are unlinked
for N segregating genes, how many phenotypes and genotypes in F2?
2^N phenotypes, 3^N genotypes
whats medels third law
the product of law probability: multiply the probabilities of independent events to get the probability of the combined event: eg. for a dihybrid cross do a combo of two monohybrids giving a 3:1 phenotypic ratio
what patterns are there for autosomal recessive inheritance
males and females equally affected. can only express trait if homozygous recessive. if parents have, then children will have. if children have but parents dont, then parents are heterozygotes. can skip generations
what patterns exist for X-linked dominant inheritance
- all daughters of affected male will have trait
- no male to male transmission
- sons can have trait only if mother has
what patterns are there for autosomal dominant traits
- equally likely in males and females
- male to male transmission
- traits dont tend to skip gens
- if parents have trait, children have
what inheritance patterns for X-linked Recessive inheritance
- more likely in males
- if mother has trait, all sons have
- no male to male transmission
- son of female carrier has fifty % chance of being affected
give an example of recessive lethal genes
manx phenotype in cats. homozygous recessive: lethal, heterozygote: tailless
give an example of incomplete/partial dominance –> what is it?
what is genotype/phenotype ratio?
eg. flower colour of Snapdragon plant (Antirrhinum). cross red x white –> 1:2:1 ratio of red:pink:white BLENDING of dominant and recessive allele. Heterozygote has an intermediate phenotype to parents due to haploinsufficiency? PHENOTYPE RATIO = GENOTYPE RATIO
give an example of codominance –> what is it?
what is genotype/phenotype ratio?
eg. human MN blood group.
2 alleles encode different gene products, homozygote expresses both alleles
2 forms of antigenic glycoprotein M and N on surface of RBC
3 genotypes: L^M/L^M; L^M/L^N ; L^N/L^N
3 phenotypes: L^M, L^MN, L^N
1:2:1 ratio
the recessive and dominant traits appear together, eg. in patches/splodges
what is the inheritance of a single gene affected by? (Two main points)
- intra - locus gene action: how alleles interact (complete/incomplete/co dominance)
- inter-locus gene action: no interaction, epistasis, additive gene action)
give an example of where two genes affect a single trait
skin pigmentation in corn snakes: one gene determines orange pigment and other determines black pigment. they act independently. P: OObb (orange) x ooBB (black) –> OoBb (camouflage)
give example of when multiple alleles exist in a popn
how do pops vary in the frequency of these alleles?
human ABO blood system a-->21% b-16% O-->63% recessive therefore no antigens 3 alleles, 6 combos/genotypes/4 phenotypes (AB,A,B,O) I^A/I^B are codominant
what is epistasis
what is general overall result
an interaction between two genes in a pathway whereby one (epistatic) gene interferes with the phenotypic expression of the other hypostatic gene
result in fewer phenotypic classes than expected
what does a ratio totalling 16 suggest
dihybrid: 2 genes
give ratio for both dominant and recessive epistasis
DOMINANT: 12:3:1
RECESSIVE: 9:3:4
give example of dominant epistasis
12:3:1 fruit colour in summer squash:Curcurbita
W is dominant at its own locus and masks the expression of alleles @ a 2nd locus. W inhibits gene Y, if no W (ie.ww), Y not inhibited
pathway: white–>yellow–>green
give an example of polygenic inheritance
what ratio can exist?
how does it occur?
how many phenotypic classes do 2 polygenes give?
2 genes each with 2 alleles with additive gene action
eg. Kernal colour in wheat (F2 ranges from white to red –> 4 colours and white)
2 genes each with 2 alleles with additive gene action
each A or B adds 1 unit of colour, each a or b adds 0 units
2 polygenes give 5 phenotypic classes
1:4:6:4:1
define penetrance (in a popn)
the percentage of individuals exhibiting the phenotype associated with their allele
define expressivity (wrt. alleles)
the degree to which a given allele is expressed at the phenotypic level
what is variable expressivity
all individual express the phenotype but to different degrees
what is variable penetrance
not all individuals exhibit the phenotype associated with their allele
what are derivations away from the expected Mendelian phenotypic ratios caused by
sex linkage incomplete dominance codominance multiple alleles lethal genes variable expressivity variable penetrance
give an example of an autosomal recessive disease
PKU: phenylketonuria
albinism
give example of autosomla dominant
achnodroplasia: dwarfism
huntingtons (HH is v. v rare)`
give an example of incomplete penetrance of an autosomal dominant trait
polydactyly
give an example of variable penetrance of an autosomal dominant trait
type 1 osteogenesis imperfecta: brittle bone disease
varying levels of disease include: mild short stature/>10 fractures in childhood/premature hearing loss/reduced bone density
outline s ymptoms of PKU
what type of disease is it, how is it caused, incident rate
autosomal recessive
inborn errors of the metabolism
1:10000-25000
symptoms: irreversible mental retardation, smelly odour, poor feeding, eczema, reduction in melanin: pale skin/fair hair/blue eyes
caused by a mutation in a gene encoding phenylalanine hydroxylase enzyme, so phenylalanine cant convert to tyrosine, instead get phenylpyruvate
what are the main outcomes of meiosis
reduces chrom number
diploids form haploid gametic cells which are n (haploid number of chromosomes) and c (haploid DNA content)
this is so that fertilisation can restore diploidy (2n+2c)
how does meiosis restore haploidy in yeast/alga/fungus
meiosis restores haploidy in some haploids after diploid zygote is formed
how are variant gametes produced?
due to independent assortment of non homologous chromosomes and crossing over of homologous chromosomes
what similarities exist between spermatogenesis and oogenesis
both involve meiosis
both produce 4 haploid daughter cells
what differences exist between spermatogenesis and oogenesis
- TIME: sperm process begins at puberty and ends at death, ovum process begins before birth and ends at menopause
- FORMATION: sperm: each daughter cell develops into sperm, ovum: one daughter cell produces the ovum, the 3 other polar bodies die
when does male/female meiosis start?
female meiosis starts in fetus and stops at diplotene before birth
male meiosis starts at puberty
what happens to menstruated oocytes
they continue meiosis 1 but stop in meiosis 2 unless fertilised
compare meiosis and mitosis for the following: 1. occurs in which cells 2. no. divisions 3.what happens to chromosome number 4. is there an S phase? 5.does synapsis occur? 6, do crossovers occur? 7.does the centromere divide? 8. does it cause variation? 9. what is the ploidy level of daughter cell?
meiosis: ME mitosis:MI
1. MI: somatic ME:sex
2. MI:1 cell divides into 2 daughter cells ME: 2 cell divisions and 4 daughter cells
3. MI: maintain ME:halved
4. MI: one premiotic S phase per cell division ME: one premiotic S phase for both cell divisions
5. MI: normally no pairing of homologues ME: Prophase 1: full synapsis of homologues
6. MI: NO ME:Yes at least one per homologous pair
7. MI:centromeres divide at anaphase ME: centormeres dont divide at anaphase1 but do at anaphase 2
8. MI:NO ME:YES
9. MI:Diploid ME:Haploid
what is the synaptonemal complex? when does it form?
protein lattice resembling railroad tracks and connects paired homologous chroms
mediates chromosome pairing, synapsis, and recombination
there is no SC in male Drosophila as no recombination
forms during prophase 1 in zygotene
what happens prior to meiosis 1
interphase: DNA replication produces sister chromatids
what happens overall in meiosis 1
separation of homologous chromosomes
what stages are there in meiosis 1
Prophase (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis), Metaphase, Anaphase, Telophase
what are the stages in prophase1, what happens? generally
pairing of homologous chromosomes and crossing over of non sister chromatids at chiasma
Leptotene, Zygotene, Pahcytene, Diplotene, Diakinesis)
what happens in metaphase 1?
tetrads migrate to metaphase plate - after spindles formed. random alignment of tetrad
what happens in anaphase 1
segregation of homologous chroms and separate by moving to opposite sides of cell
what happens in telophase 1
cytoplasmic cleavage and cell division. chromosomes move to opposite sides of cell adn then cell divide
what happens overall in meiosis 2
segregation of sister chromatids then cell division
what happens generally in prophase 2
centromeres replicate, spindle apparatus forms
what happens in metaphase2
chromosomes line up along metaphase plate in middle
what happens in anaphase 2
sister chromatids separate and begin moving to opposite sides of cell
what happens in telophase 2
chromosomes move to opposite sides of cell and then cell divides
discuss karyotype of grasshoppers
have 8 pairs autosomal chroms and XX/XO sex chroms
2n=16 autosomes + XX/XO
what chroms do female grasshoppers have? how many
2n=16+XX chromosomes therefore 18 chromosomes
what chroms do male grasshoppers have? how many
2n=16 +X chromosomes therefore 17 chromosomes
discuss sperm formation in grasshopper
autosomes: 2 x 8+X (primary spermatocyte), goes through meiosis 1, to form 8 and 8 + X (secondary spermatocyte) then meiosis 2 to form 2 times 8 (male determining sperm) and two of 8+X spermatozoa which are female determining
what does the male grasshopper karyotype consist of?
3 large metacentrics (L1,L2,L3)
1 X chromosome acrocentrics (M4,M5,M6,M7)
1 pair short acrocentrics (S8)
what happens in leptotene 1
condensing and coiling of chromatin and chromosome becoming visible
“homology search” which precedes homologous pairing
what happens in zygotene 1
shortening/thickening and initial rough pairing ie. alignment of homologues
SC synaptometal complex begins to form
paired homologues called bivalents
why is there no SC in male drosophila?
no recombination
why can only females be used for Drosophila gene mapping
because gene mapping relies on recombination and mala drosophila dont undergo recombination
what happens in pachytene 1
condensation continues
chromosomes in the homologous pair become visible
chromatids may be visible
pairing is stronger now in SC
crossing over (physical exchange of non-sister chromatids)
define crossing over (in meiosis 1)
physical exchange of non- sister chromatids
what occurs in early diplotene 1?
SC synaptometal complex starts to break down
repulsion of homologous chromosomes extends along the length except @ chiasmata
chromatids clearly evident
what occurs mid diplotene 1
repulsion of homologues
chiasmata (interchange points)
when does human female meiosis start and stop?
in foetus and stops at diplotene before birth
what happens to menstruated oocytes
they continue meiosis 1 but stop in meiosis 2 unless fertilised
what happens in diakinesis 1
chiasmata near the ends
at least 1 chiasma holds tetrad together
spindle formation occurs
nucleolus and nuclear envelope breaks down
two centromeres of tetrad attach to spindle
what happens in metaphase 1
chromosomes of tetrad are their shortest
tetrads move onto the equator of spindle
centromeres orientated to the poles
non homologous pairs align randomly on spindle
what happens in early anaphase 1
contraction of spindle fibres drags apart the homologous chromosomes
four arms of metacentric L3 visibleX chrom is undivided and moves to one of the daughter cells
what happens in telophase 2
spindle plane occurs at a 90degree angle to first division
what is the chromosomal theory of inheritance
parallel behaviour of chromosomes and genes they contain links the process and pattern of inheritance
independent assortment of genes on different chromosomes
how are different meiotic products produced
due to random alignments of bivalents at metaphase 1 resulting in parental or recombinant genotypes
how do you work out RF values (recombination frequency)
%RF = [(no. recombinants)/total number]*100
how do you know that 2 genes are on different chromosomes (wrt. RF values)
Rf=50%
no preferential inheritance for parental arrangement of alleles
what did TH Morgan discover, how did he do this
sex linkage
tested male offspring; akin to a testcross as male offspring only inherit mums X
so looking at genes on the X chromosome
when does crossing over occur
during pachytene 1 of meiosis
what is recombinance a function of (wrt. genes)
the distance between genes
what RF values are expected for incomplete linkage
0-50%
usually
define syntenic
when genetic loci are on same chromosome
define non-syntenic
when genetic loci are on different chromosomes
are syntenic genes linked
not always - syntenic genes may or may not be linked
give an example of Y linked inheritance
hairy ears
give an example of X linked inheritance
X - linked recessive Ichthyosis
discuss cytoplasmic inheritance
mitochondrial genes and chloroplast genes are linked and show maternal inheritance
how does recombination happen for non-syntenic genes
for genes on different chromosomes the random alignment of tetrads in metaphase 1 and segregation of homologous chroms results in recombination through independent assortment
how does recombination happen in syntenic genes
for genes on same chromosome, crossing over of the homologous chroms during pachytene 1 (visualised as chiasmata) results in recombination
why dont syntenic genes obey Mendels law? which law do they not obey
genes showing linkage (syntenic) are inherited together thus they disobey Mendels second law of independent assortment - unless crossovers occur
why were Mendels experiments successful in showing that traits are passed from parent to offspring in a predictable way
- chose suitable organism (small,easy to grow/short gen time)
- pea varieties available with lots of contrasting traits (7)
- he could ‘cross’ pea plants (usually self fertilise, but he could cross-fertilise/pollinate varieties with contrasting traits
- he showed good experimental technique and scientific rigour
- the genetics were the simplest (discrete traits, determined by single genes, with two alleles at each locus)
what is complementary gene action and its ratio
• A pair of genes can often work together to create a specific phenotype. 2 different phenotypes instead of the 4. must have a dominant allele in both genes to result in the purple flower phenotype 9:7 RATIO
what is a proband
the affected individual under investigation in a human pedigree analysis
What were Morgans main insights``
1st: crossing over - visualized by chiasmata
2nd: genes are arranged linearly on chromosomes
what RF value expected for complete linkagw
50%
