HIGHER HUMAN BIOLOGY Unit 2 Physiology and health 2. Postnatal Screening

Recap of Ante-natal screening What is the purpose of antenatal screening? What are the two types of ultrasound scan? What are dating scans and chemical tests used to estimate? What are anomaly scans used for? What are biochemical tests and what are they for during pregnancy? Name two antenatal screening methods. What are the advantages and disadvantages of the two methods? What is a karyotype and what are they used to determine? What is the Rhesus antibody and what are the risks associated with this antigen during pregnancy?

What should I know? What is postnatal screening? Name a metabolic disorder that is screened for after birth? What is a pedigree chart? What are pedigree charts used to do? What is meant by autosomal recessive? What is meant by autosomal dominant? What is incomplete dominance? What is sex-linked recessive? Be able to give examples of the above forms of inheritance

What is postnatal screening? Blood tests that are completed on the new born in the weeks following birth They are used to detect metabolic disorders

Name a metabolic disorder screened for after birth PKU – phenylketonuria (Revision!!) In-born error of metabolism Must be diagnosed early, after 6 months mental deficiency will occur Affected individuals cannot process the amino acid phenylalanine PKU sufferers require a restricted phenylalanine diet to prevent mental deficiency, particularly in childhood while brain still developing and for the rest of their life

Other disorder that are screened for are... Congenital hypothyroidism – underactive thyroid due to inadequate formation of thyroid gland therefore production of thyroxine is reduced Galactosaemia – inability to process the sugar galactose effectively due to lack of enzyme to do so Other amino acid disorders

What are pedigree charts? Pedigree charts show a pattern of inheritance of a particular characteristic

What are pedigree charts used to do? Charts will predict the possible chance of an individual inheriting a characteristic

What is autosomal dominant? Trait passed on chromosomes which are not the sex chromosomes (X or Y) Characteristic appears in phenotype Recessive form is masked Recessive individuals cannot pass on the characteristic to their offspring E.g. Huntington’s disease

What is autosomal recessive? Only appears in phenotype if individual is homozygous recessive Heterozygous unaffected parents can produce affected offspring Males and females equally affected

What is incomplete dominance? When neither allele is dominant over the other in a heterozygous individual Example of incomplete dominance Sickle cell anaemia -caused by a mutation in the gene for haemoglobin -result is haemoglobin S- inefficient at carrying oxygen -To have sickle cell anaemia you must be homozygous for the sickle cell gene (SS) -People who have sickle cell anaemia suffer shortage of oxygen due to the abnormal shape of their haemoglobin -Their red blood cells are sickle shaped and are less flexible than normal red blood cells – causes blockage of blood vessels and eventually death of tissues

Examples of incomplete dominance cont’d Sickle cell Trait -These people are heterozygous for the mutant allele (HS) -do not suffer from sickle cell anaemia -Red blood cells are normal shaped -Red blood cells contain both types of haemoglobin -Suffer only slight anaemia -the mutant allele is only partially expressed = incomplete dominance Advantage of being sickle cell trait -HS is very common in some parts of Africa where malaria is also very common -Possessing the HS genotype gives you resistance to malaria -The mosquito cannot make use of the S form of haemoglobin

Sickle Cell Anaemia Pedigree Chart

What is sex-linked inheritance? Revision!! Sex chromosomes -male XY -female XX -Y chromosome is smaller than the X chromosome X Y

What does sex-linked mean? cont’d X There are 4 to be aware of: -colour blindness -haemophilia -muscular dystrophy All of these are caused by a recessive allele for the disorder present on the X chromosome Y All the genes in this area here are said to be sex-linked because there are no genes on the Y chromosome to offer dominance. Males will therefore inherit all of these genes from their mother.

Example of sex-linked conditions in humans Colour blindness -More common in males (female could have dominant form of the gene on her other X chromosome that would prevent her from inheriting the condition) -Colour blind male = X c Y no letter on the Y chromosome, no gene for colour blindness present -Carrier female = X C X c does not have colour blindness but has 1 in 2 chance she could pass it on to her son -Normal male and female = X C Y and X C X C -Colour blind female (rare) = X c X c homozygous recessive for condition

XCXcXCXc XCXCXCXC XcYXcY XCYXCY

Examples cont’d Haemophilia -caused by a mutation of blood clotting factor VIII gene -blood does not clot as well as it should -H = normalh = haemophilia -Again haemophilia is very rare in females Try the following crosses ParentFatherxMother Phenotypenormalcarrier What are the chances of a haemophiliac son? ParentFatherxMother Phenotypehaemophiliacnormal

XHYXHY XHXhXHXh XhYXhY XHXhXHXh

Examples cont’d Muscular dystrophy – Revision!! -Muscle wasting disease (caused by defective muscle protein dystrophin) -Almost entirely restricted to males -Sufferers severely disabled from an early age and normally die before they can pass the gene on to offspring, late teens/twenties (natures way)

Effect of inheritance and twins Monozygotic twins Identical – one egg fertilised by one sperm Ball of cells splits into two to produce two fetuses Genetics are identical – both will inherit conditions

Dizygotic twins Non-identical – two eggs fertilised by one sperm each Two fetuses develop in two amniotic sacs with a placenta each Genetically non-identical One twin could have a condition whereas the other would not

Activities Testing your knowledge Qs p155 No’s 1 – 3 Chapter 8 – 10 summary Practice questions on genetic problems