Transcript Breeding and Fertilisation

Standard Grade
Biology
Breeding and Fertilisation
and Genotype
By the end of this lesson:

You should be able to:
Know what a genotype is
Know what alleles are
Know the difference between homozygous
and heterozygous
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Genes come in pairs
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All the cells of an organism have TWO
copies of every gene (except sex cells).
Humans have 46 chromosomes, but these
exist in 23 pairs
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What happens at fertilisation?
Sex cells are also known as
GAMETES
 Gametes do not have 2 copies of
each gene, but only one
 When a male sex cell fuses with a
female sex cell at FERTILISATION
the ZYGOTE has 2 copies of each
gene (one copy from mum and one
from dad)

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2 copies
of each
gene
2 copies
of each
gene
Dad
Mum
1 copy of
each gene
sperm
Egg
1 copy of
each gene
Zygote 2 copies
of each
gene
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Genotypes
We said earlier that different
characteristics can have different
PHENOTYPES (this describes the
way the characteristic looks)
 The genes for a particular
characteristic can give different
instructions
 Different forms of the same gene
are called ALLELES
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Genotypes
The alleles of a gene are
represented by letters
 The DOMINANT allele is
represented by a CAPITAL letter
 The recessive allele is represented
by a SMALL letter
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Examples

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Hair colour, Blonde is recessive to brown
B = Brown allele
b = Blonde allele
Height in pea plants, Tallness is dominant
to dwarfness
T = Tall allele
t = Dwarf allele
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Genotype
For each characteristic, we have 2
alleles
 One came from Mum and the other
from Dad!
 The two alleles present in an
organism are known as its
GENOTYPE
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Summary
GENOTYPE = The genes possessed
by an individual
 PHENOTYPE = The physical
appearance of an individual
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Examples
Tallness (T) is dominant to
dwarfness (t)
 A tall pea plant could have the
genotype TT or Tt
 A dwarf pea plant could only be tt
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Examples
Brown hair (B) is dominant to blonde
hair (b)
 A brunette could have the genotype
BB or Bb
 A Blonde could only be bb
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Homozygous & Heterozygous
If the two alleles are the SAME as
each other (e.g. TT or tt) the
genotype is described as
HOMOZYGOUS.
 If the two alleles are different (e.g.
Tt) then the genotype is described
as HETEROZYGOUS
 If an individual is described at true
breeding, it means they are
HOMOZYGOUS
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Task

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Insert and Complete the “Passage of
Genes” diagram from your diagram pack
into your jotter.
Use the words “rough” or “smooth” to
describe the guinea pig coat colour.
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Task

1.
2.
3.
Answer the following questions in
sentences in your jotter
What is the phenotype of the F1 guinea
pig?
Use letters to show the genotype of
the F1 guinea pig.
From this information which phenotype
do you think is dominant?
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Can you do it?

Can you answer the following?
What is a genotype?
What are alleles?
What is the difference between homozygous
and heterozygous?
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