Higher Human Biology Subtopic 12 (b) Genetic Screening and Counselling Physiology and Health Higher Human Biology Subtopic 12 (b) Genetic Screening and Counselling

Learning Intentions To revise key terms which relate to genetics To explain what a pedigree chart is and how it is used to analyse patterns of inheritance To discuss the use of these charts in counselling

2015 Exam Essay Title A Describe hormonal control of the menstrual cycle under the following headings: events leading to ovulation; 6 events following ovulation. 4

Reminder Can you remember the key words about genetics? Haploid Diploid Gene Alleles Recessive Dominant Homozygous Heterozygous Genotype Phenotype

Reminder Genetic Crosses!

Pedigree Charts (family trees) These can be used to analyse patterns of inheritance. Phenotypes for a characteristic help construct the tree then the genotype can be worked out.

Pedigree Chart

Patterns of Inheritance Conditions linked to the sex chromosomes (X and Y) are called sex linked Conditions linked to all other chromosomes are called autosomal. There are 4 main patterns to remember : Autosomal recessive Autosomal dominant Autosomal incomplete dominance Sex linked recessive

Autosomal recessive E.g. cystic fibrosis Rarely shows up Skips generations Shows up in couples who are closely related Equally shows in male and females E.g. cystic fibrosis

Outcomes Sufferers All homozygous recessive (cc) Non Sufferers Homozygous dominant (CC) Heterozygous (Cc)

Page 152

Page 152

Page 152 – What would the councillor think? Cystic fibrosis is known in Sandra’s family and the counsellor has worked out she has a 2 in 3 chance of being a carrier. Ian’s family has no history of cystic fibrosis. Outcome Low risk of producing a child with CF

Autosomal dominant E.g. Huntington’s Trait appears in every generation Each sufferer has an affected parent Equal in males and females If a branch of the tree is missed then the trait is gone E.g. Huntington’s

Outcomes Sufferers Homozygous dominant (HH) Heterozygous (Hh) Non Sufferers - All homozygous recessive (hh)

Page 153

Page 153

Page 153 – What would the councillor think? Both are still too young to know if they have received the mutant allele from their parents, however they both have a 1 in 2 chance of having the allele. Outcome They have a high chance of passing on the allele. 1 in 2 chance if one is a carrier. 3 in 4 chance if both are!

Autosomal incomplete dominant Full condition rarely expressed (both parents must be partial sufferers) Part condition much more common Equal in males and females E.g. sickle cell

Outcomes Sufferers Non Sufferers Part sufferer = heterozygous (HS) Full sufferers = homozygous for the incompletely dominant allele (SS) Non Sufferers - Homozygous dominant (HH) for the other incompletely dominant allele

Page 154

Page 155

Page 155 Both Chika and Sabato are heterozygous for the trait and so carry the incomplete allele for sickle cell. Outcome They have a 1 in 4 chance of their child fully expressing the trait, a 1 in 2 of part expression and a 1 in 4 of their child being unaffected.

Sex-linked recessive More males than females Sons of affected males don’t show the condition as the get the Y chromosome from their father Females are the carrier of the gene E.g. haemophillia

Outcomes Sufferers Non Sufferers Homozygous recessive ( XhY and rarely XhXh ) Non Sufferers Homozygous dominant ( XHY or XHXH ) Heterozygous female carrier ( XHXh )

Page 155

Page 156

Page 156 Jane’s family has a history of haemophilia but Hamish’s does not. A counsellor would conclude that Jane has a 1 in 2 chance of being a carrier since her mother and sister are carriers. Outcome There would be a 1 in 4 chance of the child having haemophilia

Learning Intentions To revise key terms which relate to genetics To explain what a pedigree chart is and how it is used to analyse patterns of inheritance To discuss the use of these charts in counselling