Transcript mendelian genetics

MENDELIAN
GENETICS
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Monohybrid Crosses
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
Allele

Genotype

Phenotype

Homozygous

Heterozygous
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BB
B
bb
B
b
Bb Bb
Bb Bb
b
B =
b =
BB =
Bb =
bb =
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Dominance
 The
brown eye colour allele is dominant over
the blue allele
 Question

A brown eyed homozygous male and a blue-eyed female
have a child. Draw a diagram to show the colour of their
child’s eyes.
The child later marries a blue-eyed woman. What
chance would they have of having a blue-eyed child?
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

A pure-breeding purple rabbit was crossed with a
pink rabbit. All the offspring were purple.
Draw a genetic diagram to show why this was.
Steps to answering genetics questions
1.
2.
3.
4.
5.
6.
Decide on suitable letters
Work out which characteristic is dominant – the offspring
phenotype will tell you this
Write the genotypes of the parents
Draw a Punnett square
Always put the dominant feature 1st
Write down the phenotypes of each genotype and state
the numbers of each type
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Cystic fibrosis
F
= normal CTFR allele
 f = faulty CTFR allele
 CTFR
gene codes for a protein channel
allowing exit of chloride ions from cells
 Mutated CTFR protein are not placed in the
plasma membrane.
 This causes a build up of thick mucus in the
lungs and other organs.
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 Draw
a genetic diagram to determine the chance of
a heterzygous man and homozygous dominant
woman having a child with cystic fibrosis
 Explain what is wrong with each of these statements


A couple are both carriers for CF will have 4 children, 1
with CF and 3 without’
If a couple’s first child has CF, their second child will
not.
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Co-dominance
Occurs when both alleles are expressed in
the phenotype
 Example:
 A cat has 2 alleles for coat colour black (B)
or white (W).
 A cat with 2 black alleles has black fur, a cat
with 2 white has…..
 A heterozygote produce both white & black
pigments resulting in grey fur.
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Co-dominance example:
WhatAllele
is the ratio of phenotypes produced by
crossing two grey cats?
B
F
W
F
FB
FW
Gene locusFB FBFB FBFW
FW
FBFW
W
W
F F
1 Black:2 Grey:1 White
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Height inheritance in plants
HTHT= tall stem HTHS = medium stem HSHS =
short stem
 A farmer crosses 2 heterozygotes. Draw a
diagram to show the genotypes and
phenotypes of the offspring
 A farmer wishes to produce all medium
stemmed plants. Which two parents must he
choose to cross in order to achieve this? Draw
a diagram to explain your answer.

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Test Crosses
 Crosses
an organism showing the dominant
phenotype with a homozygous recessive.
These are carried out to identify if the
phenotypically dominant organism is
heterozygous or homozygous.
 If the dominant organism is homozygous all
offspring will show the dominant characteristic.
 If the dominant organism is heterozygous 50%
of the offspring will show the dominant trait and
50% of the offspring will show the recessive
trait
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Example: a dog breeder wants to find out if his black lab
is a heterozygote or a homozygote.
If all the offspring are black the breeders dog is homozygous
If only
areor
black
 Black
lab50%
– AA
Aathe dog is heterozygous
 Golden
lab – aa
a
a
A
Aa
Aa
A
Aa
Aa
a
a
A
Aa
Aa
a
aa
aa
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Test cross Question:
 A farmer
wants to find out if his fat pig is a
heterozygote (FF, Ff = fat and ff = thin). He
completes a test cross. All the offspring are
fat. What can the farmer deduce about the
parent pigs genotype? Draw a Punnett square
to explain your answer.
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Multiple Alleles
E.g. Blood groups: 3 alleles exist - IA,IB,i
 IA and IB are co-dominant
 i is recessive to IA and IB
 IAIA
Group A
 IAi
 IAIB = Group AB
 IBIB
Group B
 Ibi
 ii = Group O
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Multiple allele example
 Example:
A couple have a child. One parent is
blood group AB, the other is O. What are the
possible genotypes of their child?
 IBii or IAii
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Sex Inheritance






There is 1 pair of sex chromosomes in a diploid
cell.
All other chromosomes are known as autosomes.
In humans the sex chromosomes are called X and
Y
Girls = XX
Boys = XY
Sex chromosomes are not always homologous
(e.g. in a boy the X and Y are not homologous but
they do have homologous regions.)
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Sex linkage
 The
X chromosome contains more genes than the Y
chromosome. Therefore girls (XX) have 2 copies of
every gene; boys only have a single copy of some genes
that are carried on the X chromosome but not on the
Y.
 Example: A gene on X chromosome codes for a certain
enzyme. A faulty gene is recessive but leads to a
disease called Lesch-Nyhan disease.
 XN = normal
Xn = disease
 Possible genotypes:
 XNXN = Normal girl
XnY = Sufferer boy
XNXn =
XNY = Normal boy
Normal girl
 Xn X n =
Sufferer girl
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SEX-LINKED DISORDERS
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Autosomal linkage
 When
2 or more genes are located on
the same chromosome they tend to
be inherited together.
 Example
 Blood
group gene and nail & patella
development gene
 Both located on chromosome 9
 Very close together
Dihybrid
Crosses
Looks at the
inheritance of 2 genes
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 Example:
hair colour and eye colour
 These genes are on different
chromosomes
 Where: B = brown eyes
b = blue eyes
A = brown hair
a = blonde hair
 If a person had the genotype AaBb
they would have brown hair and brown
eyes
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 Person
with genotype AaBb
A a
 At
B
b
metaphase I the chromosomes can line up as
A
B
a
b
or
a
B
A
b
 Leading
to the following gametes in approximately
equal numbers:
AB
 This
ab
aB
Ab
is to do with independent assortment
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Question:
What gametes would be made from:
1.
aabb
ab
Aabb
ab
AAbb
Ab
4.
AABB
AB
5.
aaBb
aB
2.
3.
Ab
ab
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How to draw out a dihybrid cross
2
people met, one with the genotype AaBb and
the other with the genotype aabb. Work out
the possible offspring
Parents
AaBb
Gametes
AB, Ab, aB, ab
x aabb
ab
AaBb
Brown eyes brown hair
aB
Aabb
aaBb
Brown eyes blonde hair
Blue eyes brown hair
ab
aabb
Blue eyes blonde hair
AB
Ab
ab
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Two people with the genotype AaBb
met. Work out the possible offspring.
 Parents:

Gametes:
AaBb
x
AB, Ab, aB, ab
AaBb
AB, Ab, aB, ab
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
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There are four possible combinations of the two
characteristics. Peas can be:
1) round and yellow
2) round and green
3) wrinkled and yellow
4) wrinkled and green
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Phenotype
Genotype
Gametes
all
F1
Round,
yellow
X
Wrinkled,
green
RRYY
rryy
RY
all
all
ry
RrYy
All of the F1 generation would be
heterozygous for both characteristics,
meaning that they would all be round
and yellow.
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Phenotype
Round,
yellow
X
Genotype
RrYy
Gametes
RY, Ry, rY, ry
F2
Round,
yellow
RrYy
RY, Ry, rY, ry
We really need a punnet square for
this…
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F2
9: 3 : 3 : 1
Round
Round
Wrinkled
Wrinkled
Yellow
Green
Yellow
Green
This is the typical ratio expected
in a dihybrid cross.
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Guinea pigs, which were homozygous for long, black hair were
crossed with ones which were homozygous for short white hair.
All
the
F1
offspring
had
short,
black
hair.

(i) Using suitable symbols, draw a genetic
diagram to explain this result.[3]
(ii) Draw a genetic diagram to show the
results of interbreeding the F1 offspring.[5]


(iii) State the ratio of phenotypes expected
in the F2 offspring.[1]
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