THEME 5 MOD 3 Flashcards
How many base pairs of DNA in a human cell
over 3 million, within 23 pairs of chromosomes
pedigrees displaying transmission of disease
- males are squares
- women are circles
- symbols of affected individuals shaded
- matings displayed by single horizontal line of unrelated individuals, 2 lines if related
- matings ordered from left to right based on oldest date of birth
what is the nature of alleles during inheritance
alleles on a singular chromosomes can be unlinked and recombined
types of sex chromosomes
two types: X and Y
- females: 2 X chromosomes
- males: 1 X and 1 Y chromosome
- y chromosomes much smaller
- x and y chromosomes are non-homologous, they have no counterpoint of genes
How do X and Y chromosomes pair and seperate during meiosis
- tips of X and Y chromosome allow for this
X chromosome genes
- roughly 1100 genes
- many genes have no function related to sex determination
- all called sex-linked genes as they relate to the phenotype of the sex of an individual
Y chromosone genes
- 78 genes, 25 protein coding genes (half helping determine sex
- all called sex-linked genes as they relate to the phenotype of the sex of an individual
- sets of cascade of events leading to male phenotype
- y chromosome determines the sex of humans, 50% chance of inheriting this over a second x
red/green colour blindness
- recessive x chromosome trait
- 50% chance mother heterozygous for gene passes it onto her offspring
- female offspring will also be carriers
- Ishihara colour test determines colour blindness (pattern within red and green circles detectable by non effected individuals)
- for the colour blindness gene heterozygous women wont show blindness phenotype, only homozygous women
- any men who inherits gene from mother will display phenotype
- 50% chance of a son of mother recessive for carrying gene will be colour blind and have the colour blind allele because they only have one locus for this allele on their x chromosome
- males who inherit this gene are hemizygous, only have one locus for an allele (rules of recessive or dominance no longer apply)
Haemophilia
- x linked recessive trait with mutation on gene that codes for the proteins needed for blood clotting
- 50% chance mother passes the allele on to her childeren
- 50% chance a daughter is a carrier, 50% chance a son has haemophilia
- observed in queen victoria
- unlikely for women to express the phenotype unless they inherited the mutated gene on both X chromosomes, one from each parent
mendel’s second law of inheritance
genes sort independantly during gamete formation
linked genes
- two genes positioned close enough together on a chromosome and are often inherited together
- example: gene for haemophilia and colour blindness
features of the x chromosome
- 155 megabase or 150 million base pairs, approx. 1110 genes
- p arm (short arm) and q arm (longer arm)
-dystrophin gene: protein needed for muscle cell development
-HPRT1 gene: is mutated can cause acute arthritis - colour blindness gene
- haemophilia gene
Are linked genes always inherited together
no linkage can be broken
during prophase of meiosis homologous chromosomes line up in pairs at chiasmata or crossovers and alleles of linked genes separate during recombination events
linked genes can then be inherited independently of one another
recombination of alleles during meiosis
-positions of genes don’t change, just allele positions relative to each other
- if linked genes are far enough apart, alternate allele pairs will be generated in what’s called “recombinant chromatids” in addition to parental non-recombinant allele combinations
- if linked genes have adjacent alleles no recombination/ crossing over will occur
recombination frequency
- the closer together the linked genes, the less likely they are to crossover
- recombination frequency can therefor be used to determine the distance between two genes
linkage maps
- map relative distance between genes on a chromosome and order of genes on a chromosome
- rates of exceptions in gene linkage that result in only one of the genes being inherited give us information on the distance between those genes (because they have to be somewhat separated for recombination events to occur
- in a male where only haemophilia was inherited where the mother had both haemophilia and color blindness genes, scientists could us a linkage map to determine the difference between the two genes: 12 map units apart, or 12 million base pairs apart
high density linkage maps
use snps and markers that do not lie on coding regions help identify genetic loci that lay only a few thousand base pairs apart
then look at frequency of recurrence to determine the relative position of genes, a gene and a marker, or two markers
gene + marker recombination
two sets of genes further from markers will result in 4 combinations occuring at 25% each after recombination occurs
Two sets of genes closer to markers, one of which is mutant, result in the original combination 49% of the time each. we can then see recombination between the disease gene and marker is rare, so they must be close to each other. we can then estimate the location of the disease gene because we know the location of the associated snp
genome wide association study (GWAS)
- can look at hundreds of thousands of snps of individuals to map genes and relate snps to phenotypes
using snp association to height, researchers identified a HMGA2 which contributes to less than 1 cm of height
HMGA2 gene
- identified by a marker allele associated to height
- two C alleles= 0.8cm taller than two T alleles
- C-T allele: 0.4 cm taller
