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A104 BIology

Problem 08: My Parents and I

6

Presentation

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In Today’s Problem

• All of us exhibit different traits.

• Inherited materials passed on from parents to

offspring are responsible for the traits observed in the

offspring.

• With the programs provided, you were asked to

̶ Explain the observations and trends in the traits of the parents and offspring

̶ Extend what you have learnt to other cases where traits are being passed on.

What Do You Recognise?

• The different traits which can be passed on from

parents to offspring

• Inherited materials are passed on from both

parents to the offspring

• Inherited materials may or may not be expressed

in terms of traits observed in the offspring

What is your approach?

• What inherited materials are passed on from parents to offspring?

• What are genotypes and phenotypes?

• Which of these components represent the inherited materials? • What are the differences between genes and alleles?

• What are the observations and deductions of dominant-recessive alleles?

• What is the effect of the alleles on traits?

• What are some example of Dominant-Recessive cases? • What are the observations and deductions of incomplete

dominance alleles?

• What are the observations and deductions of co-dominance alleles?

• What is the expression of both dominant traits?

What inherited materials are passed

on from parents to offspring?

• It is commonly observed that children bear some

resemblance to their parents.

• The passing on of inherited materials from parents

to their offspring partially accounts for such

observations.

• The study of the inheritance of traits in pea plants

by a monk,

Mendel

, explains how inherited

materials are passed on from parents to their

offspring.

• An organism’s genotype or genetic make up

determines its phenotypes, or physical

appearance.

• Genotype is the type of combinations of genes

that one person possess. But it's not necessarily

the genes that will get turned on.

• Phenotype is what is expressed (what genes are

active) at the level of visualization.

What are genotypes and phenotypes?

Example:

(a) For the genotype AA, the phenotype is dark.

(b) For the genotype Aa, the

phenotype is dark. This shows that though it has the genotype for both dark and light colour, but the phenotype will not show light colour. Instead, it will show dark colour.

(c) For the genotype aa, the phenotype is light.

What are genotypes and phenotypes?

“A” represents the allele for dark colour and “a”

represents the allele for light colour. “A” represents the dominant allele and “a” represents the recessive allele.

• These inherited materials are the chromosomes

found in the nucleus of a cell.

• The portion of the chromosomes that controls a

certain trait, for example the colour of pods, is

known as a gene.

Which of these components

represent the inherited materials?

•a chromosome •nucleus

•a gene that controls a certain trait

• Genes can have more than one form.

• For example, the gene that controls the colour of the pods can have a form that produces yellow pods and another that produces green pods.

• Different forms of the same gene are called alleles. • However, even if the allele for yellow pods is present,

the offspring may not have yellow pods.

What are the differences between

genes and alleles?

•Two different forms of the gene that affects the colour of pods in

the pea plants. •Allele for green pods

•Allele for yellow pods

• Genes can have more than one form.

• For example, the gene that controls the colour of the pods can have a form that produces yellow pods and another that produces green pods.

• Different forms of the same gene are called alleles.

• The offspring from the pea plant takes an allele from each parent plant.

Example:

• All humans have a gene for hair colour but humans do not all have the same colour hair. The exact colour of our hair is determined by an allele, or combination of alleles, of the gene for hair colour.

What are the differences between

genes and alleles?

What are the observations and deductions

of dominant-recessive alleles?

Traits of Parent plants

Alleles Present in Parent Plants

Traits of Offspring Plants

Alleles Present in Offspring Plants

Plant type: Yellow yellow pods Breed

× Plant type: Green_1

green pods

Green trait of the pod is expressed instead of yellow. Why?

The allele that produces green pods dominates the allele that produces yellow pods in the pea plants. Offspring must contain inherited materials from each parent.

All green pods

GG gg

Gg Gg Gg Gg

Note:

The offspring plants belongs to Green_2 green pods.

What are the observations and deductions of

dominant-recessive alleles?

Traits of Parent Plants

Alleles Present in Parent plants

Traits of Offspring Plants

Alleles Present in Offspring Plants

Breed × Plant type: Green_2

green pods

Plant type: Green_2 green pods

3 Green : 1 Yellow

Offspring must contain inherited materials from each parent.

Based on the traits of Green_2 Parent plants, the

possible allele(s) present are

Or

Gg Gg

Gg

GG gG gg

GG

Gg

What is the effect of the alleles on

traits?

Traits of Parent Plants

Alleles Present in Parent Plants Trait of Offspring Plants

Alleles Present in the Offspring Plants

Effects of the Alleles on Traits

All green pods Breed × Green_1 green pods Yellow yellow pods Green_2 green pods Breed × Green_2 green pods

3 green pods: 1 yellow pods

•The allele that produces green pods dominates the allele that produces yellow pods.

•The allele that produces yellow pods will only be expressed in the absence of the dominant form.

•Therefore, the allele that produces green pods is dominant while the allele that produces yellow pods is recessive.

The combined effect of the two inherited alleles determines the traits that will be

expressed.

GG gg Gg Gg

Gg

Gg _{GG Gg gg}

• Jonathan shows a trait which his parents do not exhibit. Hence, the allele for sickle cell anaemia must be recessive.

• Jonathan had inherited:

• His parents must have the following:

• Possible forms of genes inherited by their children:

Legend:

N Normal form of gene (Dominant)

n Allele that causes the disease (Recessive)

What are some example of

Dominant-Recessive cases?

nn

Father Mother

Nn Nn

Nn

Normal (Carrier)

nN

Normal (Carrier)

nn

Sickle Cell

NN

Normal

• Possible forms of genes inherited by their children:

• The children of the parents have 75% chance of not getting the disease while the chance of getting the disease is 25%.

• However, there is a 50% chance that they are carriers of the disease.

What are some example of

Dominant-Recessive cases?

Nn

Normal (Carrier)

nN

Normal (Carrier)

nn

Sickle Cell

NN

Normal

¾ = 75% ¼ = 25%

Traits of the snapdragon

Forms of alleles present

Traits and alleles present in offspring

What are the observations and deductions of

incomplete dominance alleles?

Breed Red

Snapdragon

×

RR

White Snapdragon

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rr

Rr Rr Rr Rr

What are the observations and deductions of

incomplete dominance alleles?

• In this case, although the allele for red snapdragon and the allele for white snapdragon are expressed, the

offspring has an intermediate flower colour.

• Both alleles are not fully dominant over each other resulting in an intermediate trait.

• The Pink Snapdragon is the result of

incomplete-dominance.

Breed Red

Snapdragon

×

RR rr

Parents:

Pink Snapdragon

Offspring:

Rr White

Snapdragon

What are the observations and

deductions of co-dominance alleles?

Traits of the animal Forms of alleles present Traits of offspring Alleles Present in offspting

× Mixture of white

and red fur

Mixture of white and red fur

Fur Colours

• The offspring can have red, white or a mixture of red and white fur colour. • The offspring with both red and white fur is the result of co-dominance.

RR RW WR WW

Red

Mixture of red and white

Mixture of

red and white White

RW

RW

NB: Both red and white furs are dominant traits

Codominant genes do not mix like incomplete dominance. Instead they are both shown equally.

What is the expression of both

dominant traits?

• Blood type categorisation is an example of trait where we can have two or more dominant traits.

• In general, the 3 alleles that control the blood types are I_A – allele for blood type A

I_B – allele for blood type B I_O – allele for blood type O

• Both I_A and I_B are dominant allele while I_O is a recessive allele.

What is the expression of both

dominant traits?

• All blood types are possible for David’s siblings.

• The blood type AB is the result of co-dominance (concept covered under going further).

AB A B O

Possible alleles present in Father

Mother

I_BI_B I_BI_O

For David’s blood type to be O, alleles for blood type O must be present in each parent.

It is possible for parents with blood types A and B to have children with blood type O.

IAIA IAIO

I_BI_O

I_AI_O

I_AI_B I_AI_O I_OI_B I_OI_O

Learning points

• Based on Mendel’s observations of pea plants, certain traits are passed on from parents to offspring as a result of the passing on of two sets of inherited material, one set from each parent.

• The traits of the offspring can emerge from the inherited materials in different ways:

1. The effect of one allele of a pair is expressed in the offspring in preference to the other. (E.g. allele for green pod dominating over allele for yellow pod in the pea plants).

2. Incomplete-dominance: The expression of an intermediate trait in the offspring when one allele of a pair is not fully dominating over the other. (E.g. interaction of the allele for red flower and that for white flower gives rise to offspring with pink flowers).

3. Co-dominance: Both sets of inherited materials are dominant and are expressed simultaneously. (E.g. blood type AB is a result of the presence of an allele for type A and an allele for type B).

Discussion

A particular species of animal can have either brown or grey fur.

An animal with brown fur has either the alleles “BB” or

“Bb” for fur colour.

An individual animal with grey fur can only have alleles “bb” for fur colour.

The breeding of two parents gives rise to some offspring with brown fur and some offspring with grey fur.

Deduce all the possible pairs of alleles for fur colour of each parent. Justify your answers.