Topic 3 hard Flashcards
Law of Segregation:
When gametes form, alleles are separated so that each gamete carries only one allele for each gene
Law of Independent Assortment:
The segregation of alleles for one gene occurs independently to that of any other gene*
The law of independent assortment does not hold true for genes located on the same chromosome (i.e. linked genes)
Principle of Dominance:
Recessive alleles will be masked by dominant alleles†
† Not all genes show a complete dominance hierarchy – some genes show co-dominance or incomplete dominance
What are menders laws of inheritance?
- law of segregation
- law of independent assortment
- principle of dominance
how are gametes formed
by the process of meiosis
are gametes haploid or diploid?
haploid, this means they only possess 1 allele of each gene
the combination of alleles can be classified as:
homozygous
heterozygous
hemizygous
hemizygous
males only have 1 allele for each gene located on a sex chromosome
genotype is typically either
homozygous or heterozygous
how are human blood cells categorised?
based on the structure of a surface glycoprotein (antigen)
when will an autosomal recessive disease occur?
if both alleles are faulty
example of an autosomal recessive genetic disease
cystic fibrosis
when will an autosomal dominant genetic disease occur?
when there is one copy of a faulty allele
example of an autosomal dominant genetic disease
Huntington’s disease
genetic disease is caused by co-dominant alleles it will also only require….
1 copy of a faulty allele to occur
- However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele
a genetic disease that displays co-dominance
sickle cell anaemia
Cystic Fibrosis
Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
Individuals with cystic fibrosis produce mucus which is unusually thick and sticky
This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts
Heterozygous carriers who possess one normal allele will not develop disease symptoms
Huntington’s Disease
Huntington’s disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4
The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats)
More than 28 CAG repeats is unstable and causes the sequence to amplify (produce even more repeats)
When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration
This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years)
Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia
sex linkage
refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)
are sex-linked conditions usually X or Y linked
X linked
why are sex-linked conditions usually X linked?
very few genes exist on the shorter Y chromosome
Why do sex-linked inheritance patterns differ from autosomal patterns
Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY)
This leads to the expression of sex-linked traits being predominantly associated with a particularly gender
why are X linked dominant traits more common in females
females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous
X-linked dominant traits are more common in females (as either allele may be dominant and cause disease)
Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits
X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele
The following trends for X-linked conditions:
Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers
Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)
Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)
examples of sex-linked inheritance
red-green colour blindness
haemophilia
assigning alleles for a sex-linked trait
the convention is to write the allele as a superscript to the sex chromosome (X)
Haemophilia: X^H = unaffected (normal blood clotting) ; X^h = affected (haemophilia)
Colour blindness: X^A = unaffected (normal vision) ; X^a = affected (colour blindness)
Haemophilia
Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired
The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome
When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time
Different forms of haemophilia can occur, based on which specific coagulation factor is mutated (e.g. haemophilia A = factor VIII)
Red-Green Colour Blindness
Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues
This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome
Red-green colour blindness can be diagnosed using the Ishihara colour test
A gene mutation
a change to the base sequence of a gene that can affect the structure and function of the protein it encodes
how can gene mutations be caused
can be spontaneous (caused by copying errors during DNA replication) or induced by exposure to external elements
what is a pedigree?
a chart of the genetic history of a family over several generations
pedigree chart features
Males are represented as squares, while females are represented as circles
Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they are unaffected
A horizontal line between man and woman represents mating and resulting children are shown as offshoots to this line
Generations are labeled with roman numerals and individuals are numbered according to age (oldest on the left)
how can Dominant and recessive disease conditions may be identified?
if certain patterns occur (otherwise it cannot be confirmed) - (maybe can be determined through using pedigree charts)
