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Who am I?
B1h
OBJECTIVES
Grade G-E
• Analyse human characteristics to determine those caused
by the environment, those controlled by genes and so
inherited & those which are a result of both
Grade D-C
• State that gender (in mammals) is determined by sex
chromosomes: XX (female) and XY (male)
• Explain that genetic variation can be caused by: mutations
(changes to the genetic code); gamete formation;
fertilisation
• Recall that inherited diseases are caused by faulty genes
• State the causes of mutations
Environmental differences
Clearly, not all differences are due to just our genes…
Variation due to
inheritance only
Variation due to
environment only
Variation due to a bit
of both
Boy or Girl?
X
Y
X
“Allele”
Girl
XX
XY
Boy
Mother
Boy or Girl?
Son
Father
Daughter
Eye colour
In eye colour the brown eye allele is dominant, so we call it
B, and the blue eye is recessive, so we call it b:
BB
Bb
bb
Homozygous
brown-eyed
parent
Heterozygous
brown-eyed
parent
Blue-eyed parent
What would the offspring have?
Eye colour
Example 1: A homozygous
brown-eyed parent and a
blue-eyed parent:
X
BB
Parents:
Gametes:
Example 2: 2 heterozygous
brown-eyed parents
bb
Bb
X
Bb
B
B
b
b
B
b
B
b
Bb
Bb
Bb
Bb
BB
Bb
bB
bb
(FOIL)
Offspring:
All offspring have brown eyes
25% chance of blue eyes
Eye colour
Example 3: A heterozygous brown-eyed
father and a blue-eyed mother:
Bb
bb
B
b
b
b
Bb
Bb
bb
bb
Equal (50%)
chance of
being either
brown eyed or
blue eyed.
Another method
Example 3: A heterozygous brown-eyed
father and a blue-eyed mother:
Father
B
b
b
Bb
bb
b
Bb
bb
Mother
Inherited diseases
1) Cystic fibrosis – a disease that causes thick and sticky mucus to coat
the lungs, gut and pancreas. It’s caused by recessive alleles:
Ff
X
Ff
2) Huntingdon's disease – a disease of the nervous system that causes
shaking and eventually dementia. It’s caused by a dominant allele:
Cc
X
cc
3) Sickle cell anaemia – a disease that alters the shape of red blood cells,
thereby reducing their oxygen capacity, causing weakness and anaemia.
It’s caused by recessive alleles:
Ss
X
Ss
Haemophilia in the royal family
Worksheet B1h2