Inheritance
Standard Grade Biology
B- What is inheritance?
Where does variation come from?
Your genes…
Chromosome
comprising 2
chromatids and
a centromere
Revise
Investigating cells topic C- cell division
Animal survival topic B- reproduction
Where do you come from?
Genetic information is
passed from one generation to the next
by sexual reproduction
You receive two pieces of genetic information
about each characteristic
one from each parent
they may both be the same or different
The outward appearance of a characteristic is
called the phenotype.
Identify examples of phenotypes of the same
characteristic:
Pupil Activity
Give a definition for ‘phenotype’
Textbook p184-185. Answer Key Questions p186
Phenotypes
Human
Hair type- wavy, straight
Tongue rolling- roller, non-roller
Pea plant
Height- tall, dwarf
Seed shape- round, wrinkled
Seed colour- yellow, green
Fruit fly
Wing length- long, short
Drosophila
Eye colour- red, white
Making sex cells
Each body cell has two matching sets of chromosomes.
The reduction of the number of chromosomes to a
single set occurs during gamete formation.
Sex cells are called gametes
Sperm Egg / ova
(pollen) (ovules)
Textbook p189 Sex cell production, fertilisation
Fertilisation
Each sex cell
sperm carries one set
of
egg chromosomes,
(ovum)
hence, one set
of genes
The nuclei of sperm and egg fuse to form a zygote
with a complete set of chromosomes, forming new,
unique individual. ie.
the complete double set of
chromosomes (2 of each) is
achieved at fertilisation.
Key Questions (p189)
1. What are chromosomes and where are they
found?
2. How many matching sets of chromosomes are
present in a normal human body cell?
3. By what other words are all types of sex cells also
known?
4. Compare the number of sets of chromosomes
present in a cell before and after gamete formation
5. Name and briefly describe the process by which a
zygote receives a double set of chromosomes.
Boy or Girl?
The sex of a child is
determined by specific
chromosomes called X and Y
chromosomes.
In humans, each male gamete may have an X or a Y
chromosome, while each female gamete has an X
chromosome.
Female Male
Body cells are XX Body cells are XY
gametes
All X X or Y
Mother Father
XX XY
gametes
X X X Y
1:1 ratio
Offspring: XX XY XX XY male to
Male or female
female? F M F M offspring
This explains how the sex of a child is
determined by X and Y chromosomes.
This can also be shown X X
and combinations
worked out using a X XX XX
Punnett square:
Y XY XY
Its not just about sex!
Certain characteristics are determined by genetic
information received from the parents.
Give examples, from animals and plants:
Animals- tongue rolling, red hair
Plants- leaf shape, flower colour.
This genetic information (coded in DNA) is organised
into genes and hundreds of genes are organised into
chromosomes. You have 23 pairs, one chromosome
coming from each parent.
Hence each body cell has 2 copies of a gene. They
may be the same, or different, versions of it (alleles).
DNA
DNA is twisted into a
double helix, where
bases pair up
Every 3 bases along
the strand codes for a
particular amino acid.
These are built up into
proteins.
Hence, DNA controls
the proteins that are
made in every body cell
For interest only
Extracting DNA
DNA from a single cell can
be 2 metres long, but is so thin
that it can hardly be seen with a
powerful microscope.
When DNA is
released from cells it
clumps together to form
strands, which are
colourless and jelly like.
Extraction of DNA from cells is the first step in
many experiments in genetic engineering. DNA
can be extracted from both plant and animal cells.
Kiwi fruit Equipment
White tile
Scalpel
Tap water
Warm water 600C
Washing up liquid
Ice cold ethanol
Coffee filters
1 reel of fuse wire
Plastic measuring
cylinders
Funnels
250ml beaker
Boiling tube
Boiling tube bung
Method – Stage 1
DNA extraction mixture:
Mix 10ml washing up liquid + 3g salt +
100ml water in a 250ml beaker
Finely chop the kiwi fruit and place in
boiling tube.
Add DNA extraction mixture
Place bung on top and shake carefully.
Leave for 15 minutes at 60oC
Method – Stage 2
Filter the mixture through the coffee filter into
a 100ml beaker to separate the chopped kiwi
fruit from the clear liquid (the DNA is invisible
as it is dissolved in the clear liquid).
Transfer 5 ml of the clear liquid into a clean
test tube.
Add 5ml of cold (4oC) absolute alcohol down
the side of the tube.
DNA will appear as “fluffy” white solid.
Final “tricky” stage
The DNA can be pulled out using a fine wire
Genes are parts of chromosomes.
Working out the genes
We can predict the genetic combination of offspring
and their appearance by looking at the alleles present
and working out the various combinations.
Weblinks
Who Am I? Science
Museum, London
http://www.sciencemuseum
.org.uk/exhibitions/genes/in
dex.asp
Genes
A characteristic is controlled by the two forms of
a gene present in an individual.
(but the particular combinations are limited by the
genetic makeup of your parents).
The different forms of a gene are called alleles.
Blood type calculator Different frequencies of
genes in the population:
http://www.blood.co.uk/
pages/e13basic.html A- 42%
B- 10%
O- 44%
AB- 4%
Genetics Buzzwords
physical appearance the set of genes (alleles) that
of an organism. an organism possesses
when two alleles (gene when the two alleles are
types) are identical different
allele which controls the
characteristic whenever
it is present.
BB or Bb
allele whose characteristics only show up when it
is present on both chromosomes
bb
History
Gregor Mendel 1822-1884
Born Austria (now Czech
Republic), studied at
Augustine monastery in Brno,
and at university in Vienna.
Failed his teaching exams &
returned to the monastery for
the rest of his life.
After about two years Mendel began his investigation into
variation, heredity and evolution in plants, studying the
common garden pea, Pisum
Between 1856 and 1863 he cultivated and
tested at least 28 000 pea plants, self-
pollinating and individually wrapped each plant
to prevent insect pollination.
He analysed 7 pairs of seeds from each plant
for comparison of: shape of seed, colour of
seed, tall stemmed and short stemmed and tall
plants and short plants. He coined the words
dominance and recessiveness.
He published his work in 1866 which formed the
basis of the Laws of Heredity.
Work largely ignored, became Abbot in 1868 and
stopped his studies after breeding some good
honey, but vicious, bees! When he died (1884) the
new Abbot burned all his papers.
Work rediscovered in 1900s, significance in 1920s
Determining characteristics
In pea plants, tallness (T) is
dominant to dwarfness (t)
A tall pea plant could have a
genotype of:
TT homozygous dominant
Tt heterozygous
Both combinations have the same
phenotype- tall.
A dwarf pea plant could only have the genotype tt
Phenotype- dwarf. homozygous recessive
Gene combinations
Identify generations as P, F1 and F2 from given
examples of crosses.
The simplest genetic cross involves one
characteristic and is called a monohybrid cross.
In this the parents (P) are crossed (bred) with each
other, and the offspring (F1 generation) observed
and counted.
Parents Phenotypes Tall (Bred Dwarf
with)
Genotypes TT tt
X
Gametes all T all t
F1 genotype Tt
F1 phenotype all tall
Tall and dwarf peas
When both alleles are the same type (homozygous)
they are called ‘truebreeding’
The phenotypes of the F1 in a true-breeding cross
are uniform.
But if the tall parent had the alleles Tt (heterozygous):
Parents Phenotypes Tall Dwarf
Genotypes Tt X tt
Gametes T and t all t
F1 genotype Tt and tt
F1 phenotype 1 tall : 1 dwarf
Cross the F1 generation
The next step involves crossing two of the F1
generation to give F2 offspring. For the truebreeding
cross:
F1 phenotype tall F2 T t
F1 genotype Tt T TT Tt
gametes T and t t Tt tt
F2 generation will give 3:1 ratio of tall to dwarf
(dominant to recessive alleles) if large numbers
are bred together.
This ratio tells you that the original parents must have
been truebreeding, one for the dominant gene, the
other for the recessive gene.
Long-horned cattle
Parents
Black x black
F1 generation
Is black coat colour or white coat colour
dominant? Why?
If the parents shown are part of the F1 generation,
what do we call the generation to which their offspring
belong?
Albinism
Sam is an albino- Mary is Sam’ wife- her
his hair is white hair is dark brown
Three of their four
children have brown hair,
but Ann is white.
What is the dominant phenotype?
Will the genes for Peggy, one of the brown-haired
children both be for brown hair, or will one be for
brown, the other for white?
Explain why you chose this combination.
Punnett squares worksheet
Pupil Activity
Collect the above worksheet for pea plants
seed coat colours.
Work through the question.
and read textbook p184-185. Answer Key
Questions on p186
Practice Question
Work out the
possible
genotypes and
Parents phenotypes of
Phenotype the offspring.
genotype
F1
Phenotype
genotype
Backcross
Another method to work out if a parent is
homozygous or heterozygous for a dominant trait is
to carry out a backcross.
This is when you breed the unknown genotype
with a homozygous recessive organism
The ratios of offspring produced will identify the parental
combination of alleles
See textbook p193
BB or Bb mouse- handout
Identify examples of truebreeding, dominant and
recessive characteristics from the numbers and
phenotypes of given crosses.
What you should be able to do:
C- Explain monohybrid crosses in terms of
genotypes.
C- Predict the proportions of the phenotypes of the
F2 offspring of a monohybrid cross.
C- State that the parents in experimental
monohybrid crosses are usually truebreeding and
show different phenotypes of the same
characteristic.
Observed vs predicted
C- Explain differences between observed and
predicted figures in monohybrid crosses.
Monohybrid crosses with truebreeding parents and
F1 generation selfcrossed always produce an F2
generation with a 3:1 phenotypic ratio.
However there is often a small difference between
the observed and expected 3:1 ratio. It is not
exactly the same as it relies on chance.
The more times you do it, the closer
it comes to the exact ratio.
Sex- 50:50 chance of boy or girl
each time, but can still end up with
all the offspring the same gender!
Quick Test
1. What is a homozygous genotype?
A genotype where both alleles are the same (TT or tt)
2. What is a heterozygous genotype?
A genotype the alleles are different (Tt)
3. What is a monohybrid cross?
When 2 plants are bred together varying in only 1 characteristic
4. How many types of gamete do you get from a
heterozygous tall plant? 2 types (T and t)
5. If two heterozygous tall plants are crossed, what is
the ratio of offspring in the F1 generation?
3: 1 tall to dwarf
6. What is the genotype of a human male? XY
7. What is the genotype of a human female? XX
8. Each time a couple have a child, what are the
chances that it will be a female? 50%