KA 4: Ante- and postnatal screening (a) Antenatal screening (c) Pedigree charts (b) Postnatal screening

Learning Outcomes State 2 reasons pedigree charts are used Name the 4 patterns of inheritance that a pedigree chart can show Identify characteristics for each pattern of inheritance Draw, analyse and interpret pedigree charts over 3 generations Be able to use standardised human pedigree nomenclature and symbols sex, matings, siblings, affected individuals, twins, heterozygotes, carrier of sex-linked allele and deceased Calculate probability of outcomes in single genes inherited conditions Identify genotype for potential offspring and for parents

Key Words (revision!) Gene: a sequence of DNA that codes for a particular protein Allele: a different form or different version of a gene Phenotype: the physical appearance of an organism Genotype: The genes an organism possesses Dominant: an allele that is always expressed even if only one dominant allele is present Recessive: an allele that is always masked by the dominant allele, unless paired with another recessive allele Homozygous: 2 alleles that are identical (BB or bb) Heterozygous: 2 alleles that are not identical (Bb) A human cell contains 23 pairs of chromosomes. The first 22 chromosome pairs are called AUTOSOMES The last pair of chromosomes are called SEX CHROMOSOMES (X or Y)

In order to be successful at Genetics based questions: Fore-warning: In order to be successful at Genetics based questions: It’s all about PRACTICE! The more examples you do, the better you will be at them! Do your working! Do not just guess. Some questions may require you to do 4 punnet squares – SO DO THEM!

Pedigree Charts: Why use them? A pattern of inheritance about one characteristic can be obtained and used to construct a pedigree chart. Once the phenotypes are determines, the genotypes can also be found. Construction of a pedigree chart is carried out by genetic councillor who can advise the a couple of the chances of them passing a genetic disorder on to their children

Pedigree Charts Types of inheritance (single gene disorders): Autosomal recessive Cystic fibrosis / Thalassaemia Autosomal dominant Huntington’s chorea Autosomal incomplete dominance Sickle cell disease & sickle cell trait Sex-linked recessive Haemophilia / Duchenne Muscular Dystrophy Females who carry 2 recessive alleles suffer from the disease. Females who carry 1 recessive alleles are known as carriers. Males need only 1 recessive allele to suffer from the disease.

Another sex-linked disorder…

Different types of colour-blindness Normal colour vision uses all three types of light cones correctly and is known as trichromacy reduced sensitivity to green light reduced sensitivity to red light reduced sensitivity to blue light

Sex-linked genes Genes that are found on the X chromosome but have no homologous allele on the Y chromosome are called SEX LINKED genes Sex linked genes always show in the phenotype as there are no genes on the Y chromosome to counteract them. Sex-linked genes Missing area X Y

Pedigree Charts - Patterns Autosomal recessive Rare / may skip generations / trait expressed amongst cousins Autosomal dominant Appears in every generations / each sufferer also has an affected parent / males and females express trait equally / if 1 branch does not express the trait, it will not reappear in future generations Autosomal incomplete dominant Fully expressed form of disorder occurs rarely / partly expressed form occurs more frequently / each fully expressed sufferer has 2 parents that shows partial expression / males and females express trait equally Sex-linked recessive More males are affected than females / none of the sons of an affected male show the trait

Nomenclature &Symbols Normal female Normal male Affected female Affected male Carrier

Nomenclature &Symbols

Task: Use the previous notes on “Pedigree charts - patterns” to help you answer the MCQ exam questions…

Pedigree Charts - Patterns Answer = B

Pedigree Charts - Patterns Answer = C

Pedigree Charts - Patterns Answer = B

To determine GENOTYPES: Some Nat 5 examples  To determine GENOTYPES: Fill in the recessive alleles first Then complete the dominant alleles by looking at the CHILDREN

What is the genotype for R? What is the genotype for S? The allele for free earlobes (E) is dominant to the allele for fixed earlobes (e) What is the genotype for R? What is the genotype for S? Ee ee

Hh HH/Hh The allele for red hair (h) is recessive to non-red (H) Work out what genotype the dad must be. What 2 possible genotypes will the sister have? Hh HH/Hh

Now Higher Levels Qs!

Autosomal recessive Answer = A

Autosomal dominant Answer = B

Autosomal dominant Answer = C

Autosomal Incomplete Dominance Answer = A

Blood groups Phenotype Possible Genotype Blood group A AA or AO Blood group B BB or BO Blood group O OO Blood group AB AB

Autosomal Incomplete Dominance Answer = A

Sex-linked recessive Question 1: What are chances of a normal female and a colour-blind male having a colour blind daughter? Question 2: If the daughter of the above cross had a child with a normal, what are the chances of having a colour-blind son?

normal female x colour-blind male Sex-linked recessive normal female x colour-blind male XnY XNXN P Gametes XN or XN Xn or Y XN Xn XN Xn Y XN Y F1 genotype Carrier girl: Carrier girl: Normal boy: normal boy F1 phenotype

carrier female x normal male Sex-linked recessive carrier female x normal male XNY XNXn F1 Gametes XN or Xn XN or Y XN Xn XN XN XN Xn Y XN Y Xn Y F2 genotype Normal girl: Carrier girl: Normal boy: affected boy F2 phenotype

Sex-linked recessive Answer = D

Sex-linked recessive Answer = C

Task: Pedigree Charts Collect a pedigree chart sheet Complete the chart by filling in the genotypes for every family member Identify the type of pedigree chart, using your notes You have 15 minutes! All answers can be found in the textbook (pages 149-153)

Learning Outcomes State 2 reasons pedigree charts are used Name the 4 patterns of inheritance that a pedigree chart can show Identify characteristics for each pattern of inheritance Draw, analyse and interpret pedigree charts over 3 generations Be able to use standardised human pedigree nomenclature and symbols sex, matings, siblings, affected individuals, twins, heterozygotes, carrier of sex-linked allele and deceased Calculate probability of outcomes in single genes inherited conditions Identify genotype for potential offspring and for parents