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Chapter 17: Genetic Crosses
Gametes
All body cells except reproductive cells are called somatic cells e.g. cheek cells, liver cells etc.
Somatic cells are diploid (2n) and they have a paired set of chromosomes.
To form new organisms diploid cells undergo a type of cell division called meiosis to form gametes.
The gametes in humans are the sperm & egg. These fuse in sexual
reproduction to form the fertilised egg.
The zygote normally goes to form a new organism
We have two versions of most genes. These are called alleles. We get one gene
(one allele) from each parent. The second gene is a backup in case there is a
mistake on the other gene. If both genes have a mistake the person has a
genetic disorder.
Genes have a dominant version (given a capital letter
e.g. A) and a recessive version (given a small letter e.g.
a)
Dominant is a gene that is stronger than the
other gene. It is expressed and stops the other
other gene (allele) from working. A capital letter
is always used for a dominant gene.
e.g. BB or Bb
e.g. bb
The genotype has 2
letters used to
represent the genes
e.g. BB, Bb or bb
The phenotype is what an
organism physically looks
like e.g. black or white coat
Genotype + environment = phenotype
B = Black coat
Parent phenotypes
Parent genotypes
Gametes
Offspring Genotypes
Offspring phenotypes
b = White coat
G = Green pod
G = Yellow pod
Parent phenotypes
Parent genotypes
Gametes
Offspring Genotypes
Offspring phenotypes
Parent phenotypes
Parent genotypes
Gametes
Offspring
Genotypes
Offspring
phenotypes
S = Straight Wing
s = Curved wing
Parent phenotypes
Parent genotypes
Gametes
Offspring Genotypes
Offspring phenotypes
G = grey body
g = black body
Parent Genotypes
Gametes
Progeny genotypes
Progeny
phenotypes
Ratio
Incomplete Dominance
Parent
phenotypes
Parent
genotypes
Gametes
Offspring
Genotypes
Offspring
phenotypes
Parent phenotypes
Parent genotypes
Gametes
Offspring Genotypes
Offspring phenotypes
Pedigree Studies
A pedigree is a diagram showing the genetic history of a group of
related individuals
Sex Determination
The nucleus of each normal body cell
(somatic cell) has 46 chromosomes ie. 2n
= 46
This consists of 44 non sex
chromosomes called autosomes and two
sex chromosomes.
The autosomes control features which
are independent of whether a person is
male or female e.g. eye colour.
The two sex chromosomes are called the
X and Y chromosomes. They contain
genes that control the gender (sex) of a
species.
The X chromosome is longer than the Y
chromosome.
Humans
Every individual somatic
(non sex) cell nucleus has
two sets of chromosomes.
If they have XX the
person is female and XY
they are male.
Note: The male
determines the sex of a
child
The ratio of males to
females should be 1:1
The work of Gregor Mendel
Known as as the father of genetics.
Investigated the inheritance of 7
characteristics of peas such as flower
colour, stem height etc.
Carried out numerous experiments on pea
plants
Removed pollen producing structures from
flowers and transferred pollen into flowers
by hand.
Covered treated flowers with brown paper
bags to prevent any more pollen reaching
them.
Collected seeds from plants, grew plants
and examined them to see if they had
inherited characteristics he was looking for.
Mendel’s First Cross
Mendel’s First Law: Law of Segregation
In this law mendel predicted that a process had occurred
which would halve the number of genes (i.e. meiosis).
After fertilisation two gametes fuse to form a single cell
called a zygote. Each gamete contains one factor (allele)
for each characteristic. So each off spring receives one
factor from each parent.
Chromosomal Basis of Mendel’s First Law
Monohybrid & Dihybrid Cross
Monohybrid cross involves the study of a single characteristic e.g.
eye colour
Dihybrid cross involves the study of two characteristics at the same
time. E.g. plant size (tall or small) and pod colour (green or yellow)
Mendel’s Second Law (Law of Independent Assortment)
The word alleles can be used instead of factors
Chromosomal Basis for Mendel’s Second Law
T = tall plants
G = green pods
t = small plants
g = yellow pods
Parent genotypes
gametes
Progeny genotype
Progeny phenotype
percentage
B = Black coat
S = Short hair
b = white coat
s = Long Hair
Parent Genotypes
Gametes
F1 Phenotype
Percentages
b)
P = Purple
S = Short
p = Red
s = Long hair
Parent Genotypes
Gametes
F1 Progeny Genotypes
F1 Progeny Phenotypes
a)
Percentages
Parent Genotypes
Gametes
F2 Progeny
Phenotypes
Percentages/ratio
Answers:
A)
B)
b)
Parent
Genotypes
Gametes
F1 Progeny
Genotype
F1 Progeny
Phenotype
Percentage
The ratio of offspring in linked
crosses
Ratio of the genotypes in linked
crosses is different from those in
non linked crosses.
Sex Linkage
The Sex Chromosomes in
Humans are the X and Y
Chromosomes.
X Chromosome carries a lot of
genes and the Y chromosome is
much shorter and only carries a
few genes.
Examples of sex – linked Characteristics
1. Colour-blindness
Normal individuals can detect 3
colours of light (red, green & blue).
The allele for normal vision is
dominant (N). Colour blindness (n) is
usually an inability to distinguish red
from green.
The gene for colour vision is located on the X Chromosome (X linked)
Females can have 3 distinct genotypes with respect to colour vision
Genotypes are represented as XXNN, XXNn, XXnn
For a female to be colour-blind she must have the genotype XXnn. The
incidence of colour-blindness in females is very low at 0.2%
Males have one allele for colour vision and
this allele is on the X chromosome. The Y
chromosome has no allele for colour vision.
Only two genotypes are possible in males i.e.
XNY- or XnYMales only need the recessive allele in order to
be colour blind.
Males are more likely to be colour blind than
females.
Incidence of colour blindness in males is 8%.
Most of these males have a degree of colour
blindness. Complete colour blindness is very rare
in Ireland.
Haemophilia
Bleeding disorder caused by a lack of a particular
blood protein.
Symptom is frequent bleeding often into joints.
Without treatment haemophiliacs may bleed to
death even after a small cut.
Haemophilia is caused by a gene located on the
X chromosome. Male with haemophilia has
genotype XYnAnd female with haemophilia is XXnn. Individuals
with genotype XXNN or XXNn or XYN- do not
have haemophilia
Non – Nuclear Inheritance
Most DNA and genes are located
in the nucleus.
Non-nuclear or extra-nuclear
genes are present as small
circles of DNA in mitochondria
or chloroplasts.
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