Transcript 2.5 Variation and Inheritance

Multicellular Organisms
Key Area 2.5
Variation and Inheritance
Variation
Learning Intentions:
•To learn about variation
Success criteria:
• Define the term ‘continuous variation’ with examples
• Define the term ‘discontinuous variation’ with examples
• Define the term ‘allele’
Variation
• Differences that occur between
individual members of a species are
called variation.
• Variation can be seen in animals, for
example shell colour in clams.
• Variation also exists in plants species,
for example petal colour or carrot
length.
Variation
• During fertilisation, genes from each
parent are passed onto their offspring.
• This process creates an organism that is
genetically different from all other
members of the species.
• Sexual reproduction contributes to
variation within a species.
Types of variation
• There are two different types of variation:
• Discrete or discontinuous variation – is
where a characteristic fall into distinct
groups.
• An example is the ABO human blood group
system.
• Every human can be placed into one of the
blood groups A, B, AB or O, not in between.
Types of variation
• The second type of variation is called
continuous variation.
• This is where a characteristic can have any
value in a range, for example height in humans.
• Humans can have a range in height from
around 60cm to 250cm once fully grown.
Variation task
From the list below, categorise the characteristics into
discrete or continuous variation.
seed shape in peas (round or wrinkled)
body mass in humans
milk yield in cattle
wing length in fruit flies (long or short)
length of index finger in humans
ear lobe type in humans (attached or unattached)
eye colour in fruit flies (red or white)
body length in trout
tongue rolling ability in humans
resting heart rate in humans
Discrete versus continuous variation
Discrete variation
Continuous variation
Discrete versus continuous variation
Discrete variation
Continuous variation
seed shape in peas
body mass in humans
wing length in fruit flies
milk yield in cattle
ear lobe type in humans
length of index finger in humans
eye colour in fruit flies
body length in trout
tongue rolling ability in
humans
resting heart rate in humans
Examples of discreet variation
tongue rolling ability
ear lobes attached or unattached
presence or absence of a widow's peak
Presentation of data
• The data obtained from
a survey of a
characteristics that
show discrete variation
is normally presented as
a bar graph.
• Each distinct group is
represented by a bar.
• There is a space
between each bar.
Presentation of data
• A graph of human height
usually takes on the
form of a histogram.
• The entire range of the
characteristic is divided
into groups (subsets).
• Most people have a
height close to the
centre of the range with
fewer very short or
very tall people.
• There are no spaces
between the bars.
Insert graph drawing task
Polygenic Inheritance
• Most characteristics shown by plants and
animals show polygenic inheritance.
• This means they are controlled by many genes
that act together.
• Polygenic characteristics show continuous
variation.
• Examples of this include height, weight, skin
colour and hand span in humans.
Key Terms
• Discreet variation – variation that is clear-cut and
observable as categories. eg. blood groups in humans.
• Discontinuous – alternative term for discrete
variation, which is variation that is clear cut and
observable as categories.
• Continuous variation – when a characteristic can have
any value in a range. eg. height or weight in humans.
• Polygenic – inheritance determined by the interaction
of several genes acting together. Polygenic
inheritance shows continuous variation.
Genes
• A gene is a single piece of genetic information
made from a short section of DNA.
• A gene carries the information to code for a
particular characteristic.
• Genes exist in different forms, called alleles.
• Alleles are different versions of the same
characteristic.
• Alleles are given symbols, usually letters of
the alphabet.
Key Words
• Gene – a short section of DNA that codes for
a particular characteristic.
• Alleles – different forms of the same gene.
Allele symbols
Organism
Gene
Human
Tonguerolling
Fruit Fly
Pea Plant
Allele
Symbol
What the allele codes for in
organism
R
Allows tongue to be rolled up
from sides
r
Tongue cannot be rolled up
Wing
Length
W
Normal wing length
w
Short wings – cannot fly
Seed Colour
Y
Yellow pea seeds
y
Green pea seeds
Variation in pea plants
• Pea plants have many characteristics that can vary
between individual plants.
• Dominant characteristics are shown above the recessive
characteristics in the table.
Variation
Learning Intentions:
•To learn about variation
Success criteria:
• Define the term ‘continuous variation’ with examples
• Define the term ‘discontinuous variation’ with examples
• Define the term ‘allele’
Genetics
Learning Intentions:
•To learn about genetics
Success criteria:
• Define the term phenotype with examples
• Define the term genotype with examples
• Define the terms homozygous and heterozygous.
Starter
Starter
Polygenic
Gregor Mendel
• Genetics is the study of variation and
inheritance and has its own language.
• In the early 19th century and Austrian
monk called Gregor Mendel studied
inheritance in garden pea plants.
• He noted that pea plants produced
yellow or green seeds and started
experiments to study how pea seed
colour was inherited.
Research
• Your teacher will show you some film clips about
Gregor Mendel.
• You will be asked to complete a research task
on his life and work.
• This may be completed in class or as homework.
Introduction to Mendel (24 mins)
Twig: Mendel and Inheritance
Phenotype and genotype
• Phenotype describes the outward appearance
of the organism and is described in words.
• For example in humans hair colour can be
blonde, brown, black or red.
Remember – the
phenotype is the
physical
appearance of the
organism!
• An organism’s genotype describes the set of
genes it possesses.
• This is represented as different letters that
refer to the different alleles for the gene.
• Dominant alleles are given a capital letter and
recessive alleles are given a lower-case letter.
• The same letter is used for the alleles of one
characteristic.
Dominant and recessive traits
• Diploid organisms have two copies of every gene.
• Alleles can be dominant or recessive.
• Dominant alleles always show up in the
appearance of an organisms, even if only one
copy is present.
• Recessive alleles only show up in the appearance
of an organism if there are two copies present
in the genotype.
Homozygous and heterozygous
• The words ‘homozygous’ and ‘heterozygous’
are used to describe the set of alleles an
organism possesses for one gene – its
genotype.
• If the alleles are the same, the organism is
homozygous.
• If the alleles are different the organism is
heterozygous.
Homozygous and heterozygous
• One gene controls seed shape in pea plants.
• The allele for round seeds (R) is dominant to
the allele for wrinkled seeds (r).
• A plant with two round alleles (RR) is
described as homozygous.
• A plant with two wrinkled alleles (rr) is also
described as homozygous.
• A plant with one round and one wrinkled allele
(Rr) is described as being heterozygous.
Key Words
• Dominant – form of a gene that always shows up in
the phenotype of the organism.
• Genotype – the alleles that an organism has for a
particular characteristic, usually written as letters.
• Heterozygous – describes a genotype in which the two
alleles for the characteristic are different.
• Homozygous – describes a genotype in which the two
alleles for the characteristic are the same.
• Phenotype – the physical appearance of an organism.
• Polygenic – inheritance determined by the interaction
of several genes acting together.
• Recessive – allele of a gene that only shows in the
phenotype if there are two copies of the allele in the
genotype.
Monohybrid cross
• Geneticists can investigate patterns of inheritance in
plants and animals by studying one characteristic at a
time.
• Crosses are set up between two true breeding
organisms that differ in only one way. eg. smooth
and wrinkled seeds in pea plants.
• The offspring produced are observed and the
different numbers of each phenotype are counted.
• This information can be used to determine which
alleles are dominant and which are recessive.
Monohybrid Cross
• A monohybrid cross is a genetic cross
between two organisms that differ in one way
only. eg. sooth versus wrinkled pea seeds
Genetics
Learning Intentions:
•To learn about genetics
Success criteria:
• Define the term phenotype with examples
• Define the term genotype with examples
• Define the terms homozygous and heterozygous.
Genetics
Learning Intentions:
•To learn about genetics crosses.
Success criteria:
• Define the use of fruit flies in genetics experiments.
• Define family trees.
Starter
Starter
Heterozygous
Recessive
Monohybrid
Fruit flies
• Geneticists use a variety of plant and animal
species to study inheritance.
• An example of an animal used for genetic crosses
is the fruit fly – Drosophila melanogaster
• This “model species” has been used by scientists
for over 100 years
• Fruit flies are usually found in warmer regions of
the world and feed on decaying fruit
• Why do you think fruit flies are used for genetic
crosses?
Twig: Fruit flies
Why are fruit flies used for
genetic crosses?
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•
•
•
Flies are small, cheap to feed and keep
Males and females can be told apart easily
Short life cycle, produce many offspring at once
Range of characteristics that can easily be identified eg.
red versus white eyes
• Have only 8 chromosomes
so limited number of genes
Viewing Drosophila
• Your teacher will show you some breeding tubes
containing fruit flies
• Flies can be anaesthetised so they can be viewed
under a microscope
• Try to identify the differences between males
and females
• Return the flies safely before they wake up!
female – pointed
abdomen
male – rounded
abdomen
Human inheritance
• Unlike pea plants, humans do not breed to suit
geneticists.
• They also produce too few offspring to allow reliable
conclusions to be drawn about phenotypic ratios.
• However, the laws of genetics still apply and
particular traits can be traced through several
generations of a family by constructing a family tree.
• A family tree is a diagram that shows how members
of human family are related to one another.
Family trees
• Males and females are given different symbols. eg.
males may be represented by a square and females by
a circle.
• Symbols are then shaded or left unshaded depending
on the phenotype of the individual.
• Individuals that produce offspring together are
joined with a horizontal line.
• A vertical line joins offspring with their parents.
• It is possible to work out the genotypes of certain
individuals in a family tree by analysing those of their
parents and siblings.
Say hello……to the Reebops
Making Reebops
• In this activity you will make “Reebops” to help
you understand how the inheritance of different
alleles leads to variation in offspring
• Reebops are imaginary animals, made from play
dough, pom poms and pipe cleaners
• They have 16 chromosomes (in 8 pairs) in their
body cells
• Follow the instructions to find out
what happens when these
organisms breed!
Genotype decoder key
Characteristic
antennae
Genotype / phenotype code
AA = 2 antennae
Aa = 2 antennae
aa = no antennae
BB = 3 body
segments
Bb = 3 body
segments
bb = 2 body
segments
tail
TT = curly tail
Tt = curly tail
tt = straight tail
nose
NN = red nose
Nn = red nose
nn = yellow nose
legs
LL = green legs
Ll = green legs
ll = yellow legs
sex
XX = female
XY = male
eyes
EE = 2 eyes
Ee = 2 eyes
ee = one eye
humps
HH = 1 hump
Hh = 1 hump
hh = 3 humps
body segments
A a
B b
E e
H h
N n
T t
L l
X X
A a
B b
E e
H h
N n
T t
L l
X
Y
Method 1 : Making gametes
1) Find your envelope containing Mum Reebop’s chromosomes and your
Dad Reebop’s chromosomes.
2) Open each envelope, take out the cards, keeping Mum’s (pink) and
Dad’s (blue) separate.
3) Sort each set of chromosomes into pairs of the same length. Dad
will have two unmatched chromosomes!
4) Now turn the cards over and randomly take one chromosome from
each pair of Mum’s chromosomes and place in a pile called ‘female
gamete’.
5) Randomly take one chromosome from each pair of Dad’s
chromosomes and place in a pile called ‘male gamete’.
Making the gametes like this is a model of inheritance – which halves
the number of chromosomes in the gamete (haploid number), so that
when gametes combine at fertilisation, the new individual has the
correct number (diploid number).
Method 2 : Fertilisation
6) Fertilise the female gamete with the male gamete by mixing
together the female and male gamete piles. This is now your
selection of ‘baby genes’ (or your zygote chromosome set).
7) Put the chromosomes you haven’t used back into their original
envelopes.
8) Sort out the chromosomes of your new individual into pairs.
You have now mixed a random selection of half the chromosomes
from one parent with a random selection of half the chromosomes
from the other parent to make a new combination. Each parent
donated the haploid number of chromosomes (8 chromosomes) to
make the diploid number (16 – now in 8 pairs again).
Method 3 : Development – from genotype to phenotype
11) Write down the letters you have obtained in the ‘genotype
and phenotype table’ for your ‘baby Reebop’. For example, if you
have one card with the letter A and another card with the
letter a, your genotype is Aa.
12) Use the ‘decoder key’ to decide what the characteristics
(phenotype) of your baby Reebop will be based on your genotype
description. For example, if is genotype is BB, it will have 3 body
segments.
13) Collect all the materials you need for your baby Reebop and
build it with the characteristics that its genes determine.
14) Put your baby in the Reebop nursery with the other
newborns!
Materials
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Green straws – Antennae
Plasticine – Body segments
Pipe cleaners – Tail
Large pompoms – Nose
Straws – Legs
Small pompoms - Humps
Discussion questions
• What do you notice about the
features of the babies?
• Are there any babies that are
identical?
• How much genetic material does
each parent provide?
• Where is this genetic material
in the parent?
• Explain how sexual reproduction
introduces genetic variation
into offspring.
Genetics
Learning Intentions:
•To learn about genetics crosses.
Success criteria:
• Define the use of fruit flies in genetics experiments.
• Define family trees.