Transcript Lesson 3- monohybrid crosses

Inheritance
Lesson 3
Lesson 3-Learning Intention
•True breeding
•F1 offspring
•F2 offspring
What we know so far..
• An organism’s PHENOTYPE is the characteristic
which is in the PHYSICAL APPEARANCE
• Each characteristic is controlled by TWO GENES.
Each copy of the gene is called an ALLELE.
• The two genes which control the characteristic is
called the GENOTYPE
True breeding
• An organism is said to be TRUE BREEDING if, when crossed
with another organism of the same strain, it always
produces offspring of exactly the same kind
What could the genotype of a true breeding organism be?
True Breeding Organisms
•Contain 2 genes which are the same for that
characteristic- HOMOZYGOUS.
•Homozygous - 2 identical alleles for any particular
characteristic (2 identical forms of the same gene).
Example
• The invisible woman has super strength- her
genotype is HOMOZYGOUS DOMINANT (SS)
• Superman is also HOMOZYGOUS
DOMINANT for super strength (SS)
• Both parents can only pass on the gene for
super strength to their offspring (as that is all
they have)
• All offspring will possess the same genotype
as their parents for super strength (SS)
Heterozygous organisms
•Organisms that have two DIFFERENT alleles are
heterozygous (one dominant and one recessive)
•The dominant gene HIDES the recessive gene
and so the dominant gene will appear in the
PHENOTYPE (appearance)
•The organism can still pass on the recessive
allele to offspring
Homozygous and heterozygous: copy
If an organism possesses 2 identical forms of
the same gene, the organism is said to be
HOMOZYGOUS and is true breeding. If an
organism possesses 2 different forms of a
gene, it is said to be HETEROZYGOUS.
Monohybrid crosses
•Only one characteristic (MONO) is
considered
•Alleles represented by letters- capital for
the dominant gene and small letter for
recessive
Example
• For example: - in peas, round seed is dominant to wrinkled
seed. This can be represented in symbols:
Round = dominant allele = R
Wrinkled = recessive allele = r
2 true breeding parents- one has wrinkled seeds and
one has dominant seeds
what is the parent’s GENOTYPE?
First generation offspring
Parents
(true breeding)
round seeds
x
wrinkled seeds
Genotype
Alleles
What would be the possible genotype of any offspring?
First generation offspring
Parents
(true breeding)
round seeds
Genotype
Alleles
x
wrinkled seeds
RR
R
rr
R
r
What would be the possible genotype of any offspring?
All F1 offspring would be round seeds Rr (heterozygous)
r
Second generation offspring
The seeds from the first generation offspring were self crossed
F1 self-cross
Genotype
Gametes
round seeds
x
round seeds
Second generation offspring
The seeds from the first generation offspring were self crossed
• F1 self-cross
round seeds
• Genotype
• Gametes
x
round seeds
Rr
R
Rr
r
R
r
What would the possible genotypes be for these offspring?
Second generation offspring
The seeds from the first generation offspring were self crossed
• F1 self-cross
round seeds
• Genotype
x
round seeds
Rr
Rr
• Gametes
R
r
R
r
What would the possible genotypes be for these offspring?
RR : Rr : Rr : rr
Phenotype ratio : 3 round seeds : 1 wrinkled seed
Sex determination
• The sex chromosomes determine the gender of
an individual
• Two types of sex chromosome- X and Y
• Males have an X and a Y (XY)
• Females have two Xs (XX)
Sex Cells
•Female sex cells (eggs) can only pass on an X
chromosome
•Male sex cells (sperm) can contain either an X
or a Y
•Fertilisation is a RANDOM process
•The egg may be fertilised by an X or a Y sperm
Sex determination
• If egg is fertilised by an X
sperm- genotype is XX and
zygote develops into a girl
• If egg fertilised by a Y
sperm- genotype XY and
zygote develops into a boy
Sex Determination: copy
The sex chromosomes (which are inherited from the parents)
determine the gender of an individual. There are 2 types of sex
chromosome; X and Y.
Males have an X and a Y chromosome (XY genotype)
Females have two X chromosomes (XX genotype)
Females can only pass on an X chromosome in their eggs. There
is a 50/50 chance of the egg being fertilised by an X or Y sperm.
Past paper questions
Past paper questions
Past paper questions
Past paper questions
Past paper questions