Transmission genetic Flashcards
Mendel’s First Law
Alleles will always segregate away from each other into
gametes.
Chromosomal Theory of Inheritance
•Chromosomes are not visible during interphase
•Genetically males contribute traits equally with females, yet males only donate a nucleus during fertilization
- chromosomes are found in the nucleus
•The segregation of Mendel’s genetic traits mirror that of chromosome separation at meiosis
-Sex determination had a chromosomal basis
Chromosome of The Fruit Fly - Drosophila
- 3 autosomes
- XX or XY
Crossing the white-eyed variant
Female red eye x male white eye F1: all red eye F2: Female all red eye, male 1/2 1/2 Female white eye x male red eye F1: Female red eye, male white eye F2: both gender 1/2, 1/2
Reciprocal cross
A Reciprocal cross is an important genetic test when assessing whether a trait is sex-linked . When performing this cross the phenotypes of the female and male parents are reversed . So if we consider body colour in flies where we have a yellow body mutant, then in one cross we would use body colour females and yellow body colour male . In a second cross we would use wildtype body colour male and yellow body colour female. If the outcome of the two crosses was the same then we would conclude that the trait is autosomal. If the outcome was different then we would conclude that the trait is sex-linked. The flies do not need to be pure breeding in these experiments.
Bridges noticed that
●Common F1 flies showing mother-son and father- daughter transmission had standard chromosome configurations
●Exceptional F1 flies showing mother-daughter and father-son transmission had abnormal sex chromosome configurations
Sex-linked traits were consistent
Sex-linked traits were consistent with the behaviour of the X chromosome
•Mother to son inheritance
•Father to daughter inheritance
Non-disjunction
◆Aberrant segregation of chromosomes during division
●Results in cells with more or fewer chromosomes than the standard set
◆Aneuploids
❖Monosomic (2n-1), trisomic (2n+1)
❖Polyploids
◆Occurs spontaneously at low frequency
◆Can be induced by drugs that affect spindle formation
Non disjuction in action
- can occur in the first or second division of meiosis
- non-disjuction at first division: AA,aa,Aa
- Non disjuction at second: AA,aa,Aa,a or A
Non-disjunction in fruit fly
White eye female x Red eye male
- female produce XX and 0 gamte
- during cross, XXX and Y0 dies
- X red eye and XXY white eye f survuve
Meiosis and Mendelian Genetics
◆Mendel showed traits are inherited in specific ways that are predicable
◆Bridges showed that these traits are on chromosomes
◆The behavior of Mendel’s genes is driven by the process of MEIOSIS which produces gametes.
Mendel’s Second Law
Alleles of separate genes will always segregate independently into gametes.
There will be nine possible genotype with 4 pheno type
9:3:3:1
Assumption of independent assortment
- S and Y are dominant
- No genetic interaction between genes
- No linkage
Linkage
●Genes on the same chromosome are physically linked
●Genes on the same chromosome may be genetically linked
-the frequency of recombinant can use to find out how far two gene are from each other
Why is male fly used in linkage
Scoring only males because they are hemizygous
- Examining event that have happened on a single chromosome from the female parent
- Phenotype equals genotype
A test cross can be used to achieve the same thing for autosomal traits
Linkage – Test Cross
two pure type is cross with each other to create a heterozygous wild type
the wild type then combine with a homozyous recessive
if the ratio is 1/4 for each off spring, the gene is unlinked. not 1/4 result in link
The relationship between distance and linkage
- the futher the two gene is from each other, the more likely they are to recombine
- gene that alway recombine with each other is called genetically linked
- maxium 50%
What is cis and trans link
AB|ab is cis Ab|Ba is trans
Map distance calculation form test cross
Map distance = 100 x Recombinants / total progeny
Linkage - Drosophila important fact
There is NO recombination in males regardless of distance
Effect of multipult cross over
Underestimate of the recombination frequency, corss over occur more than once, returning the two gene to their OG position
Three Factor Cross
The two most frequent genotype is called parental phenotype, the two least is called recombinant class the recombinant class is used to infer the order position of two gene (the odd one in the middle)
Three Factor Cross calculation
for short strand: find total of the recombinant class of the two strand/total offspring times 100 for whole strand: sum two short strand or total recombiniation+2(Double recombinant class)
Interference
◆Recombination events are not fully independent
●A crossover may inhibit the formation of a second nearby crossover or it may enhance it
●This will affect the frequency of the double crossover class in a three factor cross.
Interference is defined as I = 1 - CC
CC = Coefficient of coincidence
if the answer is positive, one event hinder another
if negative one event boost the other
Coefficient of coincidence calculate
Observed DR / Expected DR
expected DR = mu of the two strand : 100 x total off spring
Haploid and diploid in genetic analysis
●Mendel’s laws, and the process of recombination, apply equally to eukaryotic haploid organisms that undergo a sexual cycle as they do to diploid eukaryotic organisms because they undergo meiosis.
●In many instances, these haploid eukaryotes are small and have experimental advantages similar to bacteria and bacteriophage for addressing genetic questions.
