04 Genes and Inheritance

7/27/2019 04 Genes and Inheritance

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- GCSE Bitesize - Genes and inheritance

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Science

Genes and inheritance

Chromosomes are made from DNA. Genes are short

sections of DNA. Alleles are different forms of a gene. They

can be dominant or recessive. Genetic diagrams help us tounderstand the possible outcomes when parents produce

offspring. Cystic fibrosis is a disorder of the cell

membranes caused by a recessive allele. Sickle cell disease

s another example of a genetic disorder.

DNA, genes and chromosomes

DNA

DNA [DNA : The material inside the nucleus of cells, carrying

genetic information. DNA stands for Deoxyribonucleic Acid. ]

(deoxyribose nucleic acid) molecules are large and complex.

They carry the genetic code that determines the characteristics

of a living thing.

Except for identical twins, each persons DNA is unique. This is

why people can be identified using DNA fingerprinting. DNA can

be cut up and separated, forming a sort of 'bar code' that is

different from one person to the next.

Genes

A genegene: The basic unit of genetic material inherited from

our parents. A gene is a section of DNA which controls part of a

cell's chemistry - particularly protein production. is a short

section of DNA. Each gene codes for a specific protein byspecifying the order in which amino acids must be joined

together.

Chromosomes

The cells nucleus contains chromosomeschromosomes: Rod

shaped bodies found in the nucleus of cells that contain genetic

nformation (DNA). made from long DNA molecules.


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The diagram shows the relationship between the cell, its

nucleus, chromosomes in the nucleus, and genes.

Alleles

Some characteristics, such as eye colour and the shape of the

earlobe, are controlled by a single gene. These genes may have

different forms.

Different forms of the same gene are called alleleallele: One

form of a gene.s (pronounced al-eels). The gene for eye colour

has an allele for blue eye colour and an allele for brown eye

colour.

Alleles are dominantdominant :An allele that always expresses

tself whether it is partnered by a recessive allele or by another

ike itselforrecessiverecessive: Describes the variant of a gene

for a particular characteristic which is masked or suppressed in

the presence of the dominant variant. A recessive gene will

remain dormant unless it is paired with another recessive gene:

the characteristic controlled by a dominant alleledevelops if the allele is present on one or both

chromosomes in a pair

the characteristic controlled by a recessive allele

develops only if the allele is present on both

chromosomes in a pair

For example, the allele for brown eyes is dominant, while the

Nucleus, chromosome and gene


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allele for blue eyes is recessive. An individual who inherits one

or two alleles for brown eyes will have brown eyes. An individual

will only have blue eyes if they inherit two copies of the allele for

blue eyes.

Genetic diagrams

Gregor Mendel (1822-1884) studied the inheritance of different

characteristics in pea plants. He found that when he bred red-

flowered plants with white-flowered plants, all the offspring

produced red flowers. If he bred these plants with each other,

most of the offspring had red flowers but some had white. This

was because the allele for red flowers is dominant and the allele

for white flowers is recessive. Genetic diagrams help to show

how this works.

n a genetic diagram, you show all of the possible alleles for a

particular characteristic. There will be two alleles from one

parent and two from the other parent, making four altogether.You then draw lines to show all the possible ways that these

alleles could be paired in the offspring. There will be four

possible ways but some or all of them could be repeated.

ndividuals A and B have brown eyes - only individual C has blue eyes


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n genetic diagrams, the dominant allele (this allele will always

have an affect) is shown as a capital letter, while the recessive

allele (this allele will not be seen if a dominant is present) is

shown as a lower-case letter.

The alleles in the organism are the genotype. What the

organism looks like, eg red flower is the phenotype.

Offspring wi th two alleles the same are homozygous eg

FF or ff. If the alleles are different eg Ff then it is

A genetic diagram showing the outcome of Mendel's first cross. All the

offspring have red flowers, even though they carry the recessive allele

for white flowers

A genetic diagram showing the outcome of Mendel's second cross.

Three-quarters of the offspring have red flowers and a quarter have

white flowers


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heterozygous.

Cystic fibrosis

Genetic diagrams (Punnett squares)

Genetic diagrams orPunnett squares are used to show the

possible outcomes of a particular cross. A dominant allele isshown by a capital letter, and a recessive allele by a lower case

etter.

Cystic fibrosis

Cystic fibrosis (CF) is caused by a recessive allele. In the

genetic diagram below, it is written as f.

People with CF produce abnormally thick and sticky mucus in

their lungs and airways. As a result, they are more likely to get

respiratory infections. Daily physiotherapy helps to relieve

congestion, while antibiotics [antibiotics : Substances that kill

bacteria. ] can fight infection. CF also affects the gut and

pancreas, so food is not digested efficiently.

Inheriting copies of the allele

You need to inherit two copies of the faulty allele to be born withCF. If you have just one copy, you are a carrier, but will not

experience any symptoms. If two carriers have a child together,

there is a one in four chance (or 25 per cent) of it inheriting the

disorder.

The genetic diagrams below shows why.

Cystic fibrosis is an inheriteddisorder caused by a recessive

allele. This genetic diagram

shows the possible outcomes

when both parents are

heterozygous for the faulty allele.

There is a one in four chance or


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Sickle cell diseaseSickle cell disease is a recessive

condition so the sufferer has two

copies of a faulty gene. The red

blood cells of sufferers are

misshapen and can stick together

which can block blood vessels.

Sickle cell disease sufferers canbecome very tired and quickly get

out of breath. If the sickle cells block

a blood vessel, this can be fatal.

Pedigree analysis - Higher tier

Doctors can use a pedigree analysis chart to show genetic

disorders are inherited in a family. They can use this to work out

the probability (chance) that someone in a family will inherit a

condition. This is called pedigree analysis.

All the family members are mapped onto a family tree. Those

with and without a certain trait, in this case sickle cell disease,

are shown. In the diagram those with sickle cell disease are

shown by blue shaded symbols squares for males and circles

for females. In the diagram a family tree is shown for the chance

25 percent of the offspring being homozygous for the faulty

allele, and so having cystic fibrosis.

This genetic diagram shows the

possible outcomes when only one

parent carries the faulty allele.

There is no chance of the

offspring being homozygous for

the faulty allele, and so having

cystic fibrosis.

Sickle cells


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of inheriting sickle cell disease. The doctors will not know for

certain if someone will inherit and will recommend further tests if

necessary.

1. Alfie and Esme both have sicke cell disease.

2. Of their four children, three have sickle cell, one does not

(Thomas).

3. One child, Abi who has sickle cell disease, has children

with Theo, who has normal cells.

4. They have four children. Two have normal cells and onehas sickle cells. Doctors can work out the chance of Bob

having sickle cells.

Note: you wil l not be asked to work out actual probabilities

from pedigree diagrams, but wi ll need to know how to do

this with Punnett squares.

A pedigree analysis


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Now try a Test Bite.

Back to Revision Bite

Question

What is the probability that Bob will have sickle cell disease?

Answer

If we assume that Theo is a carrier, the chance of Bob havingsickle cell disease is 50%.
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04 Genes and Inheritance

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