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Bio 113 Lecture 3
Linkage and Recombination and Deviations from Mendelian Ratios
23
Biology
Undergraduate 1
01/16/2013

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Term
which genes does independant assortment take place?
Definition
genes on different chromsomes - like homologous pairs at metaphase plate
Term
crossing over takes part on which genes?
Definition
genes on the same chromsome - like sister chromatids
Term
what is a recombinant phenotype?
Definition
where the combination of phenotypes differs from that found in either of the parents
Term
name the 2 ways recombinant phenotypes can occur through
Definition

independant assortment (genes on different chromsomes)

crossing over (genes on same chromosme)

Term

recombination frequency is the % of progeny that inherit a combination of alleles that differs from either parent.

 

How do you work it out?

Definition

Recombination frequency = 

 

number of recombinants/total number of progeny x 100%

 

Term
in fruit flies (drosophila melanogaster) how is the normal allele (wild-type) written?
Definition
as a superscript + sign
Term
if 2 genes are very close together do they have a high or a low chance of crossing over?
Definition
lower chance
Term
if gene A and B are further apart what are the chances of crossing over?
Definition
the chnaces of crossing over are high and likely
Term

name one way the distance of genes on a chromosome is depicted..

 

what is the disadvantage of this method and name an alternate method

Definition

chromosme mapping.

 

disadvantage that it leads to underestimates of the distance between 2 loci due to multiple crossovers.

alternate method - the 3 point cross

Term
when counting the number of progeny in a cross test what do the largest and the smallest numbers indicate?
Definition

largest numbers indicate parental types

 

smallest numbers suggest double cross overs

Term
in a double heterozygote there are two possible arrangement of the alleles. what are they?
Definition

coupling heterozygote (bvg+) (b vg)

 

repulsion heterozygote (b vg+) (b+ vg)

Term
there are six main factors that cause deviation from mendelian ratios. what are these?
Definition
Sex-linkage, incomplete dominance, co-dominance, pleiotrophy, polygenic inheritance and epistatis
Term
where was sex-linked inheritance first observed?
Definition
drosophila fruit flies. where the Y chromosome lacks a gene and only requires one copy from the the female X chromosome to display a phenotype.
Term
when a dominant allele does not completely mask out the effect of a recessive allele at the same locus this is known as?
Definition

incomplete dominance

 

eg. snapdragon flower. genes are RR=red and rr=white

but the F1 hybrid has an intermediate phenotype of pink

Term
what happens to the ratio of the phenotypes in F2 when incomplete dominance is involved?
Definition
the results show a modified 3:1 ratio...more like 1:2:1
Term
what is co-dominance?
Definition

where each allele affects the phenotype in seperate, distuinguishable ways.

 

AB alleles in blood group are said to be co-dominant

Term
whats the difference between incomplete dominance and co-dominance?
Definition

incomplete dominance - blending of traits..diluted phenotype

 

co-dominance - no blending: traits appear together and remain distinct in the heterozygote. eg. both A and B antigens on the RBC

Term
where a single gene has multiple effects on phenotype it is said to be...
Definition

pleiotrophic.

eg. in a pea, the gene that determines flower colour also affects seed colour

eg. humans that are heterozygous for sickle cell are also resistant to malaria

Term
where a single trait is determined by multiple genes is known as?..
Definition

polygenic

eg, continous variation in the population, height, weight and skin pigmentation

Term
when one gene masks or modifies the expression of another gene it is said to be.......... to that gene
Definition

epistatic

 

epistatis is greek for stoppage

Term

true or false.

if gene A is epistatic to gene B then gene A can mask the phenotype conferred by gene B?

Definition

this is true..we say gene A is epistatic to gene B

here B is known as the synthesis gene and A is known as the depostion gene

Term
what is the epistatic gene also known as?
Definition
the depostion gene...the one that it masks is called the synthesis gene
Term
how does epistatis affect the outcome of a dihybrid cross?
Definition
it modifies the 9:3:3:1 ratio to 9:3:4 
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