HHB physiology and health - antenatal and postnatal screening

describe the role of antenatal screening

antenatal screening identifies the risk of a disorder so that further tests and a prenatal diagnosis can be offered

name the type of screening used for dating and anomaly scans

the type of screening used for dating and anomaly scans is ultrasound imaging

explain role of dating and anomaly scans

dating scans take place between 8 and 14 weeks, they determine the stage of pregnancy and due date, this is used with tests for marker chemicals (blood and urine tests) to find out if the foetus may have any medical conditions

anomaly scans take place between 18 and 20 weeks and they may detect serious physical abnormalities in the foetus

explain why routine blood and urine tests are carried out throughout the pregnancy

routine blood and urine tests are carried out throughout pregnancy to monitor the concentrations of marker chemicals e.g. HCG and AFP

explain how false positive results can occur

measuring a chemical at the wrong time could lead to a false positive result
for example, high levels of HCG can indicate down's syndrome however, levels of HCG increase then steadily decrease therefore testing too early could lead to a false positive

give an example when a diagnostic test would be offered

an abnormal chemical concentration can lead to diagnostic testing to find out if the foetus has a medical condition

describe how to carry out CVS

foetal cells are collected from a sample of cells taken from the placenta and used to produce a karyotype

describe how to carry out amniocentesis

foetal cells collected/obtained from a sample of amniotic fluid and used to produce a karyotype

compare the advantages and disadvantages of CVS to amniocentesis

CVS can be carried out earlier in pregnancy than amniocentesis however, there is a higher risk of miscarriage

while amniocentesis has a lower risk of miscarriage than CVS but is carried out later in the pregnancy

name the diagram produced which shows the chromosome complement of a person

this is called a karyotype and it shows an individual's chromosomes arranged as homologous pairs, this
can be used to detect chromosome abnormalities and determine the sex of the baby

describe how karyotypes are produced

cells from samples can be cultured (grown) to obtain sufficient (enough) cells to produce a karyotype which can diagnose a range of conditions

define the terms allele, dominant, recessive, homozygous, heterozygous, carriers, genotype, phenotype, autosomes and sex chromosomes

allele - different forms of a gene (B,b)

dominant allele - always shows in a person's phenotype if present (BB or Bb

recessive allele - only present in a person's phenotype if two recessive alleles are inherited (bb)

homozygous - two of the same alleles present in a person's genotype (bb or BB)

heterozygous - two different alleles present in a person's genotype (Bb)

carriers - a person who has a recessive allele for a condition but does not show any symptoms

genotype - the alleles a person has for a trait or characteristic (BB, Bb, bb)

phenotype - the physical expression of the alleles of a trait or characteristic (brown eyes or blue eyes)

autosomes - any chromosome which is not a sex chromosome

sex chromosomes - a chromosome which determines the sex of a person (XX- female, XY- male)

P (parents) - parent generation

F1 - first generation (offspring of the parents)

F2 - second generation (offspring of the first generation)

explain how a heterozygous individual proves which characteristic is dominant

a heterozygous individual proves a characteristic is dominant as they are heterozygous meaning they must carry both forms of the gene and their genotype still presents as that characteristic

describe incomplete dominance

incomplete dominance causes heterozygous individuals to have a blend of the two homozygous traits

describe inheritance patterns of sex-linked recessive single gene disorders

sex linked characteristics are determined by alleles found on the X sex chromosome
females - carry two alleles one on each X chromosome

males - only carry one allele as they have one X chromosome and there is no allele carried on the Y chromosome

explain why males are more likely to inherit a recessive sex-linked disorder

they receive the allele on the X chromosome from their mother so if they have the recessive condition, their mother must be a carrier if they are unaffected

they receive a Y chromosome from their father meaning they cannot receive an affected sex-linked allele from their father

males are more likely to show the phenotype for recessive sex-linked disease as they only need to inherit one recessive allele to have the recessive phenotype

use family trees to analyse inheritance patterns shown by autosomal recessive, autosomal dominant, incomplete dominance and sex-linked gene disorders

autosomal recessive - when an individual needs to inherit two recessive alleles to show the characteristic

autosomal dominant - when an individual needs to inherit one dominant allele to show the characteristic

incomplete dominance - in some cases dominant allele does not fully express itself, this causes heterozygous individuals to have a blend of the two homozygous traits

sex-linked gene disorders - sex-linked disorders affect more males than female, if more males are affected than females, then it indicates that a condition might be sex-linked
sex linked characteristics are determined by alleles found on the X sex chromosome

explain why predicted and observed ratios are often different

the predicted ratio is different to the observed ratio as fertilisation is a random process

describe how postnatal screening can diagnose PKU

new born babies are tested for metabolic disorders for example phenylketonuria (PKU) and cystic fibrosis

explain why individuals with PKU are placed on a restricted diet

in PKU, phenylalanine cannot be broken down by its normal pathway so instead is broken down into toxic compounds which can cause mental disabilities and seizures, therefore individuals with high levels of phenylalanine are placed on a restricted, low protein diet