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Heredity. Heredity. Genes control the characteristics of living organisms. Genes are carried on the chromosomes. Chromosomes are in pairs, one from each parent. Genes are in pairs. Genes controlling the same characteristics occupy identical positions on corresponding chromosomes. - PowerPoint PPT Presentation
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Heredity
Genes control the characteristics of living organisms
Genes are carried on the chromosomes
Chromosomes are in pairs, one from each parent
Genes are in pairs
Genes controlling the same characteristics occupyidentical positions on corresponding chromosomes
The gene pairs control one characteristic
But they do not always control it in the same way
Of the gene pair which help determine coat colorin mice, one might try to produce black fur and its partner might try to produce brown fur
The gene for black fur is dominant to the genefor brown fur
Dominance
The genes are represented by letters
The gene for black fur is given the letter B
The gene for brown fur is given the letter b
BB bb
The genes must have the same letter but thedominant gene is always in capitals
Symbols
The genes of a corresponding pair are called alleles
This means alternative forms of the same gene
B and b are alleles of the gene for coat color
B is the dominant allele
b is the recessive allele
Alleles
A black male mouse (BB) is mated (crossed) with afemale brown mouse (bb)
In gamete production by meiosis, the alleles areseparated
Sperms will carry one copy of the B allele
Ova will carry one copy of the b allele
When the sperm fertilizes the ovum, thealleles B and b come together in the zygote
F1
B
BB
B
b
b
b
b
B
b
meiosis
meiosis
fertilization
All offspring willbe black (Bb)
sperm mother cell
ovum mother cell
zygote
The offspring from this cross are called the F1 (First Filial) generation
They are all black because the allele for black coat color isdominant to the allele for brown coat color
These Bb mice are called heterozygotes. Because the B and b alleles have different effects; producing either black or brown coat color The mice are heterozygous for coat color
The BB mice are called homozygotes because the two allelesproduce the same effect. Both alleles produce black coats.
The bb mice are also homozygous for coat color. Both allelesproduce a brown coat color
The next slide shows what happens when the two heterozygotes are mated and produce young
B
b
B
b
B
b
B
b
B
B
B
b
B
b
b
b
BB
Bb
Bb
bb
sperm mother cell
ovum mother cell
meiosis
Possible combinationsFertilizationsperms
ovazygotes
A neater way of working out the possible combinations is to use a Punnett Square*
b
B
B b
Draw a grid
Enter the alleles in the gametes
Enter the possible combinations
female gametes
malegametes
BB Bb
Bb bbThese are the F2 generation
Punnett square
The offspring are in the ratio of 3 black to 1 brown
Although the BB and Bb mice look identical, the Bb mice will notbreed true. When mated together there is a chance that 1 in 4 of theiroffspring will be brown
This is only a chance because sperms and ova meet at random
A litter of 5, may contain no brown mice; in a litter of 12, you mightexpect 3 brown mice but you would not be surprised at anything between 2 and 5.
The total offspring from successive matings of the heterozygotes would be expected to produce in something close to the 3:1 ratio
For example, 6 successive litters might produce 35 black and 13 brown mice. This is a ratio of 2.7:1, near enough to 3:1
3:1 ratio
The offspring of the heterozygotes are the F2 generation
The genetic constitution of an organism is called its genotype
The visible or physiological characteristics of an organism are called its phenotype
The phenotype of this mouse isblack. Its genotype is BB
BB
The phenotype of this mouse is also black, but its genotype is Bb
Bb
The phenotype of this mouse is brown. Its genotype is bb
bb
Some terminology
These tobacco seedlings are the F2 generation from a cross Between heterozygous (Cc) parents. C is the gene for chlorophyll.cc plants can make no chlorophyll. There are 75 green seedlings present.What is the ratio of green to white seedlings? What ratio would you expect?
There are 21 white seedlings. This is a ratio of 75:21 or 3.57:1
Is 3.57:1 near enough to 3:1 ?*
1 CC 2 Cc and 1 cc, a ratio of 3 green to 1 white seedling
You would expect the cross to produce 72 green to 24 white seedlings (3:1)
c
C
c
C
cc
CC Cc
Cc
In most populations of animals there are approximately equal numbers of males and females
This is the result of a pair of chromosomes; the sex chromosomescalled the X and Y chromosomes
The X and Y chromosomes are a homologous pair but in many animals the Y chromosome is smaller than the X
Females have two X chromosomes in their cells.Males have one X and one Y in their cells
At meiosis, the sex chromosomes are separated so the the gametesreceive only one: either an X or a Y.
Sex chromosomes
X
Y
X
X
X
Y
X
X
X
X
X
X
X
Y
X
Y
sperm mother cell
ovum mother cell
meiosisfertilization
female
female
male
male
Sex ratio
Very few human characteristics are controlled by a single gene
Characteristics such as height or skin color are controlled byseveral genes acting together
Those characteristics which are controlled by a single geneare usually responsible for inherited defects (see slide 19)
Single gene effects
An exception is the inheritance of the ABO blood group
The IA allele produces group A The IB allele produces group B
The IO allele produces group O IO is recessive to IA and IB
The group A phenotype can result from genotypes IAIA or IAIO
The group B phenotype can result from genotypes IBIB or IBIO
The group O phenotype can result only from genotype IOIO
The AB phenotype results from the genotype IAIB
The alleles IA and IB are equally dominant (co-dominant)
ABO blood groups
Cystic fibrosis (recessive) Glands of the alimentary canal produce a thick mucus which affects breathing, digestion and susceptibility to chest infection
Achondroplastic dwarfism (dominant)The head and trunk grow normally but the limbs remain short
Albinism (recessive) Albinos cannot produce pigment in their skin, hair or iris
Polydactyly (dominant*) an extra digit may be produced on thehands or feet
Sickle cell anaemia (recessive)The red blood cells becomedistorted if the oxygen concentration falls. They tend to block small blood vessels in the joints
Genetic defects
If the genotypes of the parents are known, it is possible to calculate the probability of their having an affected child (i.e. one with the defect)
For example if a male achondroplastic dwarf marries a normalwoman, what are their chances of having an affected child?
The father’s genotype must be Dd. (DD is not viable)
The mother must be dd since she is not a dwarf
There is a 50% probability of their having an affected child
D d
d
d
Dd
Dd
dd
ddWhat are the probabilities if both parents are affected?
Genetic counselling (Genetic defects)
If two normal parents have an affected child, they must both be heterozygous (Nn) for the recessive allele n
NN Nn
Nn
N n
N
n nn
A nn parent would have cystic fibrosis
A NN parent would produce only normalchildren
Since the parents are now known to beheterozygous it can be predicted that theirnext child has a I in 4 chance of inheritingthe disease
This chance applies to all subsequent children*
Cystic fibrosis (recessive)21
Hb = haemoglobin
HbA is the allele for normal hemoglobinHbS is the allele for sickle cell hemoglobin
A person with the genotype HbSHbS will suffer from sickle cell anemia
A person with the genotype HbAHbA is normal
The genotype HbAHbS produces sickle cell ‘trait’ because HbA
is incompletely dominant to HbS
The heterozygote HbAHbS has few symptoms but is a ‘carrier’ for the disease
Sickle cell anemia (recessive)
Heterozygous recessive individuals do not usually exhibit any disease symptoms but because their offspring may inheritthe disease, the heterozygotes are called ‘carriers’
HbA
HbA HbAHbA HbAHbS
HbAHbSHbS
HbS
HbSHbS
carriers
Similarly, individuals with the genotype Nn are carriers forcystic fibrosis
Carriers
It is sometimes possible to work out the genotypes of parents andto track the inheritance of an allele by studying family trees
= normal female = affected female
= normal male = affected male
Parents have normal phenotypes but produce
an affected child
For this to happen, both parents must have heterozygousgenotypes (Nn) for the characteristic
Family trees
If one of the parents is homozygousfor a dominant allele, all the childrenwill be affected
If one parent is heterozygous for a dominant allele and the other is homozygous recessive, there isa chance that half their children willbe affected
If both parents are heterozygous fora recessive allele, there is a chance that one in four of their children will be affected
AA
Aa aa
Aa Aa
grandparents
parents
children
cystic fibrosis
What can you deduce about the genotypes of the grandparents fromthis family tree?
marriage marriage
Cystic fibrosis is caused by a recessive gene
An affected person must therefore have the genotype nn
Since neither of the grandparents is affected, they must be eitherNN or Nn genotypes
If they were both NN, none of their children or grandchildren couldbe affected
If one was Nn and the other NN, then there is a chance that 50% of their children could be carriers Nn
If one of the carriers marries another carrier, there is a 1 in 4 chance of their having an affected child
The genotypes of the grand parents must be either both Nn or oneNN and the other Nn
If both parents have the Dd genotype there is a 75% chanceof their having affected children, but the DD individual isunlikely to survive
D
D DD Dd
Dd dd
d
d
Question 1
Which of the following are heterozygous genotypes?
(a) Aa
(b) bb
(c) nn
(d) Bb
Question 2
A B C
A b c
Which of these genes are alleles?
(a) A and A
(b) A and B
(c) B and C
(d) B and b
chromosomes
Question 3
Which of the following processes separates homologous chromosomes ?
(a) mitosis
(b) cell division
(c) meiosis
(d) fertilization
Question 4
Which of the following terms correctly describes the genotype bb ?
(a) homozygous dominant
(b) heterozygous dominant
(c) homozygous recessive
(d) heterozygous recessive
Question 5
What is the likely ratio of affected children born to parentsboth of whom are heterozygous for cystic fibrosis ?
(a) 1 affected: 3 normal
(b) 3 affected: 1 normal
(c) 2 affected: 2 normal
(d) all affected
Question 6
Which of the following phenotypes corresponds to the Genotype IAIO ?
(a) Blood group A
(b) Blood group B
(c) Blood group O
(d) Blood group AB
Question 7
What is the expected ratio of offspring from a black rabbit Bb and a white rabbit bb ?
(c) 50% white; 50% black
(a) 3 black: 1 white
(b) 1 black: 3 white
(d) all black
Question 8Which of these Punnett squares correctly representsa cross between two heterozygous individuals ?
A a
A
a
A a
A
a
A a
A
a
a
A
a
AA
AA
AAaa
aa
a
Aa Aa
aa
Aa aa
aa
Aa
AA
Aa
Aa
Aa
(a) (b)
(c) (d)
Question 9
A married couple has a family of 6 boys.What are the chances that the next child will be a girl ?
(d) 1:1
(a) 6:1
(b) 1:6
(c) 3:1
Question 10
Which of the following is a ‘carrier’ genotype for a disease caused by a recessive gene ?
(a) nn
(b) NN
(c) Nn
Question 11
If normal parents have a child with cystic fibrosis
(a) one of them must be heterozygous
(b) both of them must be heterozygous
(c) one of them must be homozygous
(d) both of them must be homozygous
Answer
Correct
Answer
Incorrect