Year 10 Genetics and Variation Topic

1. Y E A R 1 0
M S G I B E L L I N I
Genetics

3. Student Learning Objectives
 Explain how variation assists in species survival
 Distinguish between continuous and discrete variation
 Explain sex chromosomes
 Distinguish between dominant and recessive alleles
 Explain the terms genotype, phenotype, homozygous and
heterozygous
 Draw and use punnet squares
 Describe what a pedigree is
 Describe DNA and genes
 Explain the genetic code
 Explain what a mutation is

4. SLO
Explain how variation assists in species
survival
Distinguish between continuous and
discrete variation

5. Uniquely you…
The population of the Earth is more than 6 billion people, and no two
individuals (apart from identical twins) are genetically the same. Why?
People are different because they inherit different characteristics
(or traits) from their parents.
Like all babies, this child carries a unique set of
genes; half from his mother and half from his
father.
A person’s unique characteristics are caused by:
 the set of genes they inherited from their parents (nature)
 the environment in which they developed (nurture).

6. Inherited and acquired characteristics
Differences in some characteristics are due to a combination of both inherited
and environmental factors. In some cases, it can be difficult to say how much
influence each factor has.
Other types of characteristics, such as
scars and hair length, are not inherited
but depend on environmental factors.
These are called acquired
characteristics.
Some characteristics, such as eye colour and
earlobe shape, are only determined by genes.
These are called inherited characteristics.

7. Nature, nurture or both?

8. Variation
 Variation occurs when
individuals within a
population differ
 Selection of those best suited
to the environment leads to
a change in the make up of a
population over time and the
evolution of new species

10. Inheritance
Why do we look the way we do?
 You inherit traits (features) from your parents
 These traits are passed to you in your chromosomes
 Organisms must have variety within populations in
order to survive change.

12. Phenotype and genotype
The overall appearance of an organism depends on two things:
The full set of genes of an organism is called its genotype.
All the observable characteristics of an organism are called its phenotype.
1. its genes (inherited characteristics)
2. the effects of the environment in which it lives.
An organism’s phenotype therefore depends on its genotype plus environmental
effects.
phenotype = genotype + environmental effects

13. Classifying variation
Characteristics can be classified in different ways. How would you categorize
variation in eye colour? Could height be categorized in the same way?
 A feature that can be measured and given a
value from a range of values shows
continuous variation.
 A feature that cannot be measured but is
one of a few distinct options shows
discontinuous variation.
Which type of variation are eye colour and height?

14. Variation
 There are two types of variation
 Continuous – range eg height
 Discontinuous – either or, you have it or you
don’t, eg tongue roller

16. Discontinuous Variation
Characteristic Yes No
A B
Male Female
Widows peak Straight edge
Free ear lobe Attached ear lobe
Straight thumb Hitch hikers thumb
Tongue roller Non tongue roller

17. Variation Graph
 What shape is your graph?
Not quite a bell shape
 If your graph does not show a bell shaped curve why
not?
Because sample size is too small
 What is the biological importance of variation?
To survive environmental change

18. What type of variation?

19. Variation
 http://www.youtube.com/watch?v=oWP3nWe-4K8
 Complete Dinosaur Variation Worksheet
 Complete page 48 Year 10 Science Workbook

20. Success Criteria
 Why is it important for populations to have
variation?
 Where do we get our inherited characteristics from?
 Give two examples of inherited characteristics
 Why don’t we look exactly like our parents?

21. Success Criteria
 Why is it important for populations to have
variation? So that they can survive environmental
changes
 Where do we get our inherited characteristics from?
Parents, parents parents, parents parents parents
 Give two examples of inherited characteristics
Nose shape, eye colour, hair colour
 Why don’t we look exactly like our parents?
Because we are a mixture of mum and dad, and the
environment

22. SLO
Describe DNA and genes
Explain the genetic code

25. Genetics the basics

26. Alleles, genes and chromosomes
Alleles – alternative form of a gene
eg blue eyes (b) or brown eyes (B)
Gene – section of DNA that codes for a
characteristic
eg eye colour
Chromosome – coiled length of DNA
(deoxyribose nucleic acid)

29. Karyotype

30. Karyotype
 Chromosomes are matched up into Homologous
Pairs (same chromosome, one from each parent)
 Homologous pairs of chromosomes are then ordered
from longest to smallest.
 Sex chromosomes always go at the bottom right
 In humans we have 22 pairs of body cells and two
sex cells – total of 46 chromosomes

33. Chromosome Numbers
 Body cells have 23 pairs or 46 chromosomes
 Egg and sperm must have half or one set of
chromosomes so that when they join to make a new
child, that child only has 46 chromosomes (not 92!)

34. Success Criteria
 Where are chromosomes found?
In the nucleus of all cells
 What is a gene?
Section of DNA that codes for a characteristic eg eye
colour
 Where do we get our inherited characteristics from?
Our parents
 How many chromosomes do we have?
46

35. SLO
Explain what a mutation is

36. Mutations
 Mutations are a permanent change in the base
sequence of DNA
 Mutations are the ultimate source of variation
as they give rise to NEW phenotypes

37. Mutagens
 Mutations (changes in base sequence of DNA
are caused by:
 UV A rays
 Radio waves
 Some chemicals
 Heavy metals
 Problems during meiosis or DNA replication

38. True or false?

39. Family resemblanceMembers of the same family
often look similar.
If the son and daughter have
children of their own one day, will
they also look like their parents?
Which parent do these children
look more like?
Humans, like all organisms, inherit characteristics from their parents. How
are characteristics passed on?
Why do members of the same family look similar?

40. Different versions of genesChromosomes in a homologous pair contain the same type of genes that code for
the same characteristics, such as eye colour.
Each different version of a gene is called an allele.
Each chromosome in the pair, however,
may have a different version of the gene.
For example, the version of
a gene on one chromosome
may code for brown eyes,
whereas the version of the gene on the other
chromosome may code for blue eyes.
allele for
brown
eyes
allele for
blue
eyes

41. Homozygous allelesIf the alleles for a characteristic in a homologous pair are the same, the
organism is said to be homozygous for that characteristic.
What colour eyes will these homozygous pairs of alleles produce?
allele for
brown eyes
allele for
brown eyes
allele for
blue eyes
allele for
blue eyes

42. Heterozygous alleles
The characteristic expressed by
heterozygous alleles will depend on
which allele is dominant and which
allele is recessive.
If the alleles for a characteristic in a homologous pair are different, the organism
is said to be heterozygous for that characteristic.
What colour eyes will this heterozygous pair of alleles produce?
allele for
brown eyes
allele for
blue eyes
?

43. Dominant or recessive?
 Dominant alleles are always expressed in a cell’s phenotype. Only one
copy of the dominant allele needs to be inherited in order for it to be
expressed. Dominant alleles (e.g. brown eyes) are represented by an
upper case letter (e.g. ‘B’).
The phenotype for a particular characteristic depends on which allele is dominant
and which allele is recessive.
 Recessive alleles are only expressed in a cell’s phenotype if two copies
of it are present. If only one copy is present, its effect is ‘masked’ by the
dominant allele. Recessive alleles (e.g. blue eyes) are represented by a
lower case letter (e.g. ‘b’).

44. What eye colour?
The allele for brown eyes is dominant over the allele for blue eyes.
The individual will have brown eyes,
because the allele for brown eyes
masks the allele for blue eyes.
allele for
brown eyes
allele for
blue eyes
So, what colour will the eyes be of an individual who is heterozygous for eye
colour?

45. Inheritance terms

46. SLO
Distinguish between dominant and
recessive alleles
Explain the terms genotype, phenotype,
homozygous and heterozygous

47. Terms;Terms;Terms;Terms
Term Definition
Phenotype Physical characteristic, blue eyes
Genotype Letters representing alleles, Bb
Homozygous Same letters (alleles) BB, bb
Heterozygous Different letters (alleles) Bb
Alleles Alternative form of a gene
Dominant Always expressed if present, capital letter, B
Recessive Requires two alleles to be present to be
expressed, lower case letter, b
Pure breeding Homozygous for the desired trait

48. Homozygous cross

49. Heterozygous cross

50. Finding the genotypeFor some characteristics, the genotype of a homozygous recessive individual can be
determined from their phenotype.
A test cross can be used to determine whether an
individual is homozygous or heterozygous for a
dominant trait.
But what about individuals that have brown
fur? Is their genotype BB or Bw ?
For example, the allele for brown fur (B) in mice is dominant over the allele for
white fur (w). This means that all white mice must therefore have the genotype
.

51. What is a test cross?During a test cross, an individual with an unknown genotype is crossed with a
homozygous recessive individual. The phenotype of the offspring will reveal the
unknown genotype.
 If all the offspring display the dominant phenotype, then the parent of
unknown genotype must be homozygous for the characteristic.
 If half the offspring show the
dominant phenotype, and half show
the recessive phenotype, then the
parent must be heterozygous for the
characteristic.

52. Using test crosses to find genotype

53. Mendel’s experimentsOver seven years, Mendel experimented on more than 28,000 pea plants!
Why were his experiments so successful?
 Pea plants grow quickly.
 Pea plants are available in pure-
breeding (homozygous) strains.
 Many pea plant characteristics show discontinuous variation; they are
either one form or another, with no intermediates. This means that their
phenotypes are easily distinguishable.

54. Mendel’s early experiments

55. Monohybrid crosses
There are two alleles controlling pea shape. This means there are three possible
genotypes that the F2 generation of plants could inherit, leading to two possible
phenotypes.
SS
ww
Sw
smooth
wrinkly
smooth
Genotype
heterozygous
homozygous dominant
homozygous recessive
Phenotype
The type of experiment that Mendel carried out, investigating just a single
characteristic, is called a monohybrid cross.
The likelihood of a trait being produced during a monohybrid cross can be
mapped out using a Punnett Square.

56. What are Punnett Squares?

57. Mendel’s laws of inheritanceAfter his research, Mendel proposed two laws of inheritance.
Mendel’s first law: the law of segregation
Mendel’s second law: the law of independent assortment
 Alternate versions of genes (alleles) cause variation in inherited
characteristics.
 An organism inherits two alleles for each characteristic – one from each
parent.
 Dominant alleles will always mask recessive alleles.
 The two alleles for each characteristic separate during gamete production.
 Genes for different characteristics are sorted independently
during gamete production.

58. Predicting Offspring
Parent Phenotype: Brown x Brown
Parent Genotype: Bb x Bb
Gametes: B b x B b
Punnet Square: X B b
B BB Bb
b Bb bb
F1 Genotype: 1 BB : 2 Bb : 1 bb
F1 Phenotype: 3 Brown : 1 Blue

59. Questions
Brown (B) eyes is dominant to blue eyes (b).
1. If two heterozygous people mate, what is the ratio of
their off spring?
2. If a homozygous brown eyed person mates with a blue
eyed person what colour eyes will their offspring have?
3. What colour eyes will the offspring of a heterozygous
brown eyed person who mates with a person with the
recessive colour for eyes be?
4. If two blue eyed people mate, what colour eyes will their
offspring have?

60. Punnet Square Rules
 Heterozygous x Heterozygous cross
= 3 dominant phenotype: 1 recessive phenotype
 Homozygous dominant cross
= All dominant phenotype
 Heterozygous X recessive cross
= 1 dominant phenotype: 1 recessive phenotype
 Recessive X recessive cross
= All recessive phenotype

61. Success Criteria
 Where do the genes for eye colour come from?
 What is the genotype of a person with brown eyes
who has a sister with blue eyes? Why?
 How can two brown eyed people have a child with
blue eyes?
 Complete page 50 of Workbook

62. SLO
Describe how gender is
determined
Explain sex chromosomes

64.  http://www.youtube.com/watch?v=8YMhkweABfQ

65. Boy or Girl
Parent Phenotype: Male X Female
Parent Genotype: XY x XX
Gametes: X Y x X X
Punnet Square: X X X
X XX XX
Y XY XY
F1 Genotype: 1 XY : 1 XX
F1 Phenotype: 1 Male : 1 Female

66. Boy or Girl
 Sex is determined by the male,
 if a sperm carrying a Y chromosome fertilizes an egg the baby
is a _______________
 if a sperm carrying an X chromosome fertilizes an egg the baby
is a _______________
 There is a 50% chance of a baby being a boy or
girl, no matter how many boys in a row are born
there will always be a 50% chance the next one
will also be a boy

67. Success Criteria
 How many pairs of chromosomes do
humans have?
 If dogs have 54 chromosomes in their body
cells, how many pairs of chromosomes do
they have?
 How many chromosomes do they have in their sex
cells?

68. SLO
Describe what a pedigree is

69. Pedigree Key

70. Pedigree Questions
1. What is the genotype of individual I2? Justify your answer
2. What is the genotype of individual III3? Justify your answer
3. What is the genotype of individual III4.?Justify your answer.
4. What is the genotype of individual III 1? Justify your answer
I
II
II
I

71. Success Criteria

72. Revision

73. Multiple-choice quiz