Transcript 10.2 AHL Dihybrid Cross and Linked Genes

10.2 Dihybrid Cross
Vocabulary
Dihybrid crosses involve two genes which
control two characteristics. There are
complications of these patterns as illustrated
in the calculations that follow.
Unlinked genes are found on different
chromosomes and can be segregated by
random assortment of meiosis/ metaphase II
Autosomal are those chromosomes other than
the XY gender determining chromosomes (not
sex linked).
IB Assessment Statement
Calculate and predict the genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal
genes.
IB ASSESSMENT STATEMENT
Distinguish between autosomes and sex chromosomes.
10.2.2 Distinguish between
autosomes and sex chromosome.
In some species there is one homologous pair
which carry genes that determine gender.
THis work has already been covered in the
section on sex chromosomes.
There are other genetic conditions (also covered
in topic 4) associated with the so called 'sex
linked" conditions.
IB ASSESSMENT STATEMENT
Explain how crossing over between non-sister chromatids of a
homologous pair in prophase I can result in an exchange of
alleles.
Crossover
The diagram shows
the loci of genes A
and B.
The alleles A is linked
to B, a is linked to b.
The homologous pairs
are held together
like this during
prophase I.
Crossover
The crossover point occurs above the
loci for gene A and below the gene
loci for A.
The position at which the exchange
occurs is called the chiasma.
The recombinants will form between
non-sister chromatids which are
crossing over.
The homologous pairs remain
attached at the chiasma until
anaphase I when they are pulled
apart.
Crossover
After anaphase II, the
chromatids are
separated.
The linked genes are:
A with B
a with b
recombinant a with B
recombinant A with b
Define linkage group.
Linked Genes
Any two genes that are found on
the same chromosome are said
to be linked.
These two genes are actually on the
same chromosome.
The A allele is linked to the B allele
along the length of DNA.
The a allele is linked to the b allele
along the length of DNA.
Linked genes do not follow
Mendel’s law of independent
inheritance
IB ASSESSMENT STATEMENT
Identify which of the offspring are recombinants in a dihybrid
cross involving linked genes.
Recombination
A recombinant refers to offspring that do NOT
have the same phenotypes as either of their
parents.
Unless the IB questions says genotypic
recombinant , then it means a different
genotype than its parents.
Unlinked Genes
Unlinked vs. Linked
Linked Genes
on a different chromosomes
On the same chromosomes.
2 genes assort into gametes into
gametes independently of each
other
Do not assort into gametes
independently of each other.
Follows expected ratios
phenotype ratios 9:3:3:1
Unexpected phenotypic ratios
Practice
In garden peas, the pairs of alleles coding for seed shape and seed
colour are unlinked. The allele for smooth seeds (S) is dominant over
the allele for wrinkled seeds (s). The allele for yellow seeds (Y) is
dominant over the allele for green seeds (y).
If a plant of genotype Ssyy is crossed with a plant of genotype ssYy,
which offspring are recombinants?
A. SsYy and Ssyy
B. SsYy and ssYy
C. SsYy and ssyy
D. Ssyy and ssYy
RECOMBINANT ANIMATION
http://bcs.whfreeman.com/thelifewire/content
/chp10/1002s.swf
Practice Question
How does recombination normally occur for
unlinked genes?
A.Crossing-over in Prophase I
B.Random chromosome assortment
C.Failure of spindles to form
D. Random gene mutations
Practice Question
Two genes A and B are linked together as shown
below. If the genes are far enough apart such that
crossing over between the alleles occurs
A
occasionally, which statement is true of the
gametes?
A. All of the gametes will be Ab and aB.
a
B. There will be 25% Ab, 25% aB, 25% ab and 25% AB.
C. There will be approximately equal numbers of Ab
and ab gametes.
D. The number of Ab gametes will be greater than the
number of ab gametes.
b
B
Practice Question
What constitutes a linkage group?
A.Genes carried on the same chromosome
B.Genes whose loci are on different autosomes
C.Genes controlling a polygenic characteristic
D. Alleles for the inheritance of ABO blood
groups
Linked Genes Practice Problems
http://www.ksu.edu/biology/pob/genetics/linke
d.htm
IB ASSESSMENT STATEMTN
Explain an example of a cross between two linked genes.
Example of Linked Genes
Using an example you have studied, explain a cross between two linked genes, including
the way in which recombinants are produced.(Total 9 marks)
Go to the below website to help you answer this question:
http://click4biology.info/c4b/10/gene10.2.htm#5
linked genes occur on the same chromosome / chromatid; genes (tend to be) inherited together / not separated / do not segregate independently; nonMendelian ratio / not 9:3:3:1 / 1:1:1:1;
real example of two linked genes;
Award [1] for each of the following examples of a cross between two linked genes. key for alleles involved in the example of a cross; homozygous
parental genotypes and phenotypes shown; F1 genotype and phenotype shown / double heterozygote genotype and phenotype; possible F2 genotypes
and phenotypes shown; recombinants identified;
over;
recombinants due to crossing over; in prophase I of meiosis; diagram / explanation of mutual exchange of parts of chromatids during crossing