Lecture 10: Genes, Chromosomes, and Human Genetics Flashcards
What makes you who you are
- your genes; found on chromosomes
Modifications to Mendelian Genetics
- genetic linkage
- sex-linked genes
- chromosomal alterations
- non traditional patterns of inheritance
1) Genetic Linkage
define genes
genes
- sequences of nucleotides in DNA
- arranged Linearly in chromosomes (we have thousands on 1 chromosome)
Do all genes assort independently
- not necessarily, even though in mendels work all 7 characters did
- some genes are inherited together because they’re on the same chromosome (2 diff genes on 1 chromosome)
- Mendel never saw this because he didn’t pick Linked genes
Linked Genes
- genes on 1 chromosome
- linked during transmission from parents to offspring (passing entire chromosome to gamete)
- inherited like single genes
Break Linkage:
- prophase 1 (meiosis 1)
crossing over: enzymatic cut and pasting can break them
Drosophila Melanogaster
Fruit Fly
- model organism for animal genetics
- compared to Mendel’s Peas
- Morgan: first genetic map
- Used to test linkage and recombination (how we unlink genes)
linkage maps= of a chromosome
- relative location of genes
How far apart genes are on a chromosome will determine
degree of linkage if they’re linked
- closer 2 genes are they more linked because the chance of crossing over decreases (crossing over unlinks)
Gene Symbolism
normal alleles (wild-type)
- usually most common allele
- designated by + symbol (wild-type common)
- usually dominant
FRUIT FLIES:
wild-type:
pr+=red eyes
vg+=normal wings
mutant:
pr=purple
vg=vestigial wings
Genetic Recombination
how will it occur?
- alleles linked on same chromosome exchange segments between homologs chromosomes
- exchanges occur while homologs chromosomes pair during Prophase 1
Experimental Evidence for Gene Linkage
Q: do purple-eye vestigial wings (pr vg) assort independently?
A: Morgan crossed wild breed with purple vestigial wings, F1 were all wild breed (dominant), then did a testcross with F1 generation and didn’t get 1:1:1:1
- HENCE NO INDEPENDENT ASSORTMENT, they’re linked
- principle of independent recombination
Recombination Frequency
- amount of recombination between 2 genes that reflects the distinct between them
- the greater the distance the greater the recombination frequency
= greater chance of crossover btwn genes
Linkage maps of chromosomes
- recombination frequencies used to determine relative borders on a chromosome
- linkage map for genes a,b, and c
8% recombination f=8 map units apart between gene a and c
a (—-8%) c (—2%) b
total: 9.6%, because its an underestimation (because of double crossing over) the real f combined is 10%
therefore 1 map unit=1% recombination
Single vs Double Crossing Over
single: two chromosomes exchange info at 1 point, leading to swapping of genes
( chromosome A has genes A1 and A2 and B has B1 and B2
leads to A1B2 and A2B1)
double: two different exchange events occur at 2 points on chromosome, more variety and combos
(so we can get more variation A1B1 and A2B2)
BECAUSE OF DOUBLE CROSSING OVER:
= sometimes the chromosome can look the same as it did before crossing over so we think 0 crossing over happened or less crossing over happened=underestimation
Can genes on the same chromosome ever assort independently
- widely separated linked genes often recombine
- seem to assort independently
- detected by testing linkage to genes between them
- Mendel looked at genes on separate chromosomes OR they were linked but so far apart that they behaved separately *
= at 50+ mpu the 2 genes behaved as if they were on 2 separate chrosomes - hence principle of independent assortment
Why only 50% or 50 MPU MAX?
(only half of the offspring show new genetic combos because of crossing over)
- double crossing over can result in the restoration of the original parental combination of alleles
- difficult to tell whether genes are on the same chromosome or not
- inheritance pattern will appear as if they are assorting independently as if they were on diff chromosomes, 50% marks this point
- double
2) Sex Linked Genes
- sex chromosomes determine sex
X and Y chromosomes in many species
XX: woman
XY: man - other chromosomes are called autosomes
Human Sex chromosomes
- Human X chromosome
*large=2350 genes (some determine sex, most determine nonsexual traits)
-many X-linked genes are nonsexual traits - Human Y chromosomes
- small (few genes)
- very little homology with X chromosome
- contains SRY gene (8 weeks post-fertilization)
- regulates expression of genes that trigger male development (females have 0 deviation)
OG DEVELOPMENT PATHWAY
- females continue
- males deviate (SRY gene)
Sex Linkage
Female (XX) 2 copies of X linked alleles
- heterozygote recessive: allele hidden (carrier)
- homozygote recessive: trait is expressed
Male (XY): 1 copy of X-linked allele, only males have Y-linked alleles
- 1 copy of a recessive allele results in expression of the trait
Eye Colour In fruit flies
-Morgans work
- Normal wild-type: RED
- Mutant: WHITE
(sex-linked)
how was the white-eye inherited?
P: RED (female) x WHITE (male)
F1: ALL RED (therefore, white is recessive)
- follows mendels laws so far…
F2: all red-eyed females,
1/2 red eyed and 1/2 white eyed males
=3/4 red;1/4 whites
= 3:1 RATIO
- female: 1:0
- male: 1:1
RECIPROCAL CROSS
- Morgan flips it to determine if sex has any influence!
P: WHITE (female) X RED (male)
F1: Females have red eyes, and males have white eyes (therefore, its X-Linked gene)
F2: he wasn’t seeing 3:1
- ratio amongst general population doesn’t line up with sex based ratio
NEW RATIO: female: 1:1
male: 1:1
1/2 red, 1/2 white
Human Sex-Linked genes
whats more common 4 males
- X-Linked recessive traits are more common in males
1) red-green colour blindness
2) hemophilia: defective blood clotting protein=mutation - Seen more commonly because they only have 1 X, females need both Xs to match to acquire a trait *
Pedigree
- show genotypes and phenoytpe
- X-Linked
1 X chromosome Inactivation
- DOSAGE COMPENSATION
- in female mammals, inactivation of 1 X chromosome in each cell makes the dosage of X-linked genes the same smalls
- Occurs during embryonic development
- because the condensation of chromosomes makes them so tight and inactivates them, its hard for them to be expressed because condensation prevents transcription/translations *
Barr Body
- Random Inactivation of either X chromosome by condensation
- Attached to side of nucleus
- Copies during mitosis but always inactive
- Results in patches of cells with different active X chromosome
Calico Cats
- Heterozygote Female (RARE to be Male)
if cat has xxi it can be male
Dominant O gene is expression turns B off
or!
Expressed; the O is mutant recessive, B is not turned off
Creates patches of orange and black fur on cat
orange: determined by inactivation of X
black: NOT determined by inactivation of X (determined by expression of O gene)
Therefore, O gene is EPISTATIC to B gene
- turning it (B) off prevents melanin deposition hence ORANGE FUR
- expression of the O gene will ctrl expression of the B gene to either produces melanin (black patch) or turning it off (orange fur)
- Chromosomal Alteration
- Deletion: broken segment lost from chromosome
missing segment contains genes that are essential for normal development or cellular functions - Duplication: broken segment inserted into homologs chromosome
- on exact same chromosome
one copy can mutate into new forms w/o srsly affecting the basic functions of organism - Translocation: broken segment attached to nonhomologus chromosome
- can also exchange DNA by accident
**generally reciprocal, so 2 non-homolgous chromosomes exchange segments, resembles genetic recbomination but the 2 chromosomes involved dont contain the same genes*
most cancers are this - Inversion: broken segment re-attached in reversed orientation (put in backwards)