CHAPTER 3 NOTES

Mendel was analyzing descendants of pure lines that differed in tow characters.

Dyhybrid
a double heterozygote such as A/a * B/b

Dihybrid crosses  A cross between two individuals identically heterozygous at two loci—
for example,
A B
/
a b
×
A B
/
a b.

Mendel’s Second Law  different gene pairs assort independently in gamete formation
o
For two heterozygous gene pairs A/a and B/b, the “b” allele is just as likely to end up
with an “A” allele.
Law applies to genes in diff. chromosomes.
o
2 alleles for a gene are packed into separate gametes
o
Alleles get back together after fertilization.

Law of independent assortment  genes on different chromosomes are transmitted
independently of one another.

Mendellian Ratios describe that equal segregation and independent assortment are
fundamental hereditary processes found throughout nature.
o
Ratios: 3:1:1:1, 9:3:3:1 and 1:1:1:1

Independent assortment on haploids
o
Fusion of parental cells result in transient diploid  meiocyte that is a dihybrid A/a;
B/b and the randomly sampled products of meiosis.
3.2.1
Geneticist can work in either of two directions:
(1) predicting the genotypes of parents by using ratios of progeny or
(2) predicting progeny ratios from parents of known genotype.
Methods of independent assortment:
(A) Punnett squares can be used to show hereditary patterns based on one gene pair, two gene
pairs, or more.

as long as 2 nonallelic genes are on diff. chromosomes:
o
They will assort independently
o
Can FOIL to find gametes (combine first, outers, inners, lasts).
(B) The branch diagram is easier to create and is adaptable for phenotypic, genotypic, or gametic
proportions.

Used for independent assorting, nonallelic genes only (genes on diff. chromosomes).
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Useful for dihybrids and up
o
Ex AaBbCC X AabbCC what is the probability will have baby AabbCC.
ASK
TEACHER !!

Use dash notation if looking for phenotypic rations only; A means AA or Aa.
(C) To simplify, we can use a statistical approach, which constitutes a third method for
calculating the probabilities (expected frequencies) of specific phenotypes or genotypes coming
from a cross:
(1)
Product Rule
 The probability of two independent events occurring simultaneously is the
product of the individual probabilities.
 The focus is on outcomes A and B
(2)
Sum Rule
 The probability that one or the other of two mutually exclusive events will occur
is the sum of their individual probabilities.
 The focus is on the outcome A
′
or A
″
.
*** IF question asks how many progeny do we need to grow? First calculate the proportion of
progeny expected to be of that genotype.
Book example=256. approach by probability of complete
failure where 11/256)=255/256
extending idea to sample size “n” = (255/256)n.
The
probability of obtaining once success is 1 minus the probability of total failure
1(255/256)n
= .95 or 95%.
**** If asking how many distinct genotypes will a cross produce?
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 Spring '07
 Gunaratne/Roman
 Genetics, Zygote

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