Chapter 14: Genetic Screening and Counselling. Higher Human Biology Unit 1: Cell Function and...

Chapter 14: Genetic Screening and Counselling.

Higher Human Biology

Unit 1: Cell Function and Inheritance

19/04/23 1Mrs Smith: Ch14: Genetic Screening and Councilling

Learning Intentions• By the end of this lesson you should be able to-

– describe Genetic screening and counselling.– Explain the use of family histories in determining genotypes.

• Examples might include albinism, Huntington’s chorea, cystic fibrosis, phenylketonuria, haemophilia and muscular dystrophy.

– Explain the use of karyotypes of foetal material where there is a possibility of genetic disorder.

– Discuss risk evaluation in cases of polygenic inheritance. – Understand post-natal screening for conditions which have a

genetic basis.

19/04/23 2Mrs Smith: Ch13: Mutations an Chromosomal Abnormalities

You need to know these words

19/04/23 Mrs Smith: Ch13: Mutations an Chromosomal Abnormalities

3

Use of Family Histories

• A pattern of human inheritance can be revealed by collecting information about a particular characteristic from the members of a family and then using it to construct a family tree (pedigree)


Types of pedigree

• There are 3 types of pedigree that you need to know about– Autosomal recessive inheritance– Autosomal dominant inheritance– Sex-linked recessive trait.

19/04/23 Mrs Smith: Ch14: Genetic Screening and Councilling

5

The Geneticist recognises Autosomal Recessive Inheritance because....

• The trait is rarely expressed• The trait tends to skip generations• The trait is expressed in some cousins• Males and females are equally affected• All sufferers of the trait are homozygous

recessive• Non-sufferers are homozygous dominant or

heterozygous.


Autosomal recessive inheritancee.g Cystic Fibrosis


The Geneticist recognises Autosomal Dominant Inheritance because....

• The trait appears in every generation.• Each sufferer has an affected parent.• When a branch of the family does not express the

trait it fails to reappear in future generations of that branch.

• Males and females are equally affected.• All non-sufferers are homozygous.• Sufferers are homozygous dominant or heterozygous.


Autosomal Dominant Inheritancee.g. Huntington’s Chorea


The Geneticist recognises Sex-linked recessive because.....

• More males are affected than females• None of the sons of an affected male show the trait• Some grandsons of affected males show the trait• All sufferers of the trait are homozygous recessive• Non-sufferers are homozygous dominantor or

heterozygous carrier females



11

Sex-linked recessive traite.g. haemophilia

Sex-linked recessive traite.g. haemophilia


FYI - Genetic code

• In 1966, the Genetic code was discovered• Human Genome Project to determine

nucleotide sequence of human DNA began in 1990 and was completed in 2003.

• Scientists are now able to predict characteristics by studying DNA. This leads to genetic engineering, genetic counseling.


Genetic code

An international team of scientists began the project to map the human genome.


Genetic Code


Genetic Code

In 1990, gene therapy was used on patients for the first time


Assessing the risk!• Once the genetic councillor has constructed

the family tree(s) and established as many genotypes as possible, he/she is in a position to assess the risk and state the possibilities.


17

Assessing the risk – Autosomal recessive. e.g. cystic fibrosis

• If a couple were considering having a family, however the female knows cystic fibrosis runs in her family but not in her partners.

• They could approach a genetic councillor.

• By analysing her family tree the councillor could work out there was a 2:3 chance that the female is a carrier.

• The councillor would already know the frequency in the British population as 1:25 for carrying the heterozygous allele for CF – this is the risk of her partner being a carrier.

• The councillor would then conclude the risk of having a child with CF as low.

Mrs Smith: Ch14: Genetic Screening and Councilling

18

Assessing the risk – Autosomal Dominant. e.g. Huntington’s chorea

• Consider a brother and sister, unlike their siblings they are too young to know if they have received the harmful allele from an affected parent.

• They could approach a genetic councillor.

• The councillor would already know the frequency in the British population, and there is a 1:2 chance that each is heterozygous allele for HC

• By analysing her family tree the councillor would then conclude the risk of having of each sibling suffering this debilitating disease as high.


19

Assessing the risk – sex-linked. e.g. Haemophilia

• If a couple were considering having a family, however the female knows Haemophilia runs in her family but not in her partners. She is anxious to know if she could pass the trait onto her sons.

• They could approach a genetic councillor.• By analysing her family tree the councillor would note that the

woman's brother and sisters son her nephew have developed this sex linked trait.

• This shows her sister and mother are carriers.• The councillor would then conclude a she has a 1:2 risk of being

carrier and a 1:4 risk that each son will be a haemophiliac.


20

Advantage and disadvantagr of Genetic counselling

• The aim of genetic counselling is to help people to make well informed decisions for themselves based on information available.

• Advantage: Analysis of a family tree allows the expert to chart he pattern of the disorder.

• Disadvantage: It is of limited value in that it can only offer an assessment of risk.


21

Pre-natal Screening

• If after assessing the risk of having baby with a genetic disorder. AND IF the couple decide to go ahead and have a baby pre-natal screening can be employed.

• Two methods of pre-natal screening depend on foetal material being obtained to allow karyotypes to be examined– Amniocentesis

– Chorionic villus sampling (CVS)


Amniocentesis• Amniocentesis is carried out

in the 18th week of pregnancy.

• It involves the withdrawal of a little amniotic fluid containing foetal cells.

• These cells can be analysed and a full karotype made.

• It slightly increases risk of miscarriage

19/04/23 23

Chorionic Villus Sampling

• A small tube is inserted into the womans reproductive tract, placental cells are removed, these cells are used for karyotyping

• ADV: CVS can be carried out at 8 weeks of pregnancy

• DISADV: Causes a higher risk of miscarriage then amniocentesis

19/04/23 24

Post-natal Screening

• At present none of the inherited disorders can be successfully treated except for PKU

• Phenylketonuria results from an inborn error of metabolism for 1:10000 b irths in Britain.

• If not detected soon after birth the baby suffers from mental retardation

• All British babies are routinely tested for excess phenylalanine after birth by means of a blood test


Testing for PKU: Guthrie's test


Task: TYK -Torrance pg106 Qu’s 1-3

19/04/23 27Mrs Smith: Ch13: Mutations an Chromosomal Abnormalities

Task: AYK -Torrance pg 106-7 Qu’s 1-4

19/04/23 28Mrs Smith: Ch13: Mutations an

Chromosomal Abnormalities

Essay Question Guide to H-Grade essays pg 64

By means o examples you have studied discuss genetic conditions of medical importance with reference to the following:– Family History– Use of Karyotypes– Post-natal Screening.

(15)

19/04/23 Mrs Smith 29

Chapter 14: Genetic Screening and Counselling. Higher Human Biology Unit 1: Cell Function and...

Documents

Transcript of Chapter 14: Genetic Screening and Counselling. Higher Human Biology Unit 1: Cell Function and...