11: Inheritance

Genetics

The scientific study of heredity.

heredity

the passing on of physical or mental characteristics genetically from one generation to another.

trait

a specific characteristic that varies from one individual to another.

gene

a sequence of DNA that codes for a protein and thus determines a trait.

alleles

alternative forms of a gene. Each trait has at least two alleles.

homologous chromosomes

A pair of homologous chromosomes carry the same genes in the same location.

• one chromosome from each homologous pair comes from the father and one comes from the mother.

• minimum of two genes for each characteristic

  • one gene is on the father’s chromosome of the pair and the other on the mother’s chromocome of the pair.

Dominant allele

• masks the other (recessive) trait

• the trait that will be expressed

• represented by a Capital Letter

Recessive allele

• the trait will only by expressed when the dominant allele is not present, and only if both alleles are recessive.

• represented by a lower-case letter.

genotype

the genetic makeup of an organism: allele combination

phenotype

the pysical characteristics of an organism: the appearance of an organism

homozygous

when an organism has two identical alleles for a particular trait. Can be dominant or recessive

Heterozygous

when an organis, has two different alleles for the same trait

Punnet square

diagram that shows the gene combinations that might result from a genetic cross. Used to calculate the probability of inheriting a particular trait.

monohybrid cross

a cross between two organisms with different alleles for one particular gene/trait

autosomal inheritance

a pattern of inheritance in which the transmission of traits depend on the presence of absence of certain alleles on the autosomes. The majority of heriditary disorders are the result of a mutated gene on an autosome. (Mutation: permanent change in DNA)

• inheritance may be dominant or recessive

• males and females are usually affected with equal frequency

autosomal dominant inheritance pattern

these diseases are frequently passed down from generation to generation. If you get just one from abnormal gene from just one parent, you can inherit the disease.

• standard dominant alleles are frequently passed on

  • heterozygous affected parent = 50% of children will be affected

  • homozygous affected parent = 100% of children will be affected

• dominant alleles that cause severe defects in people are rarely passed on, because people with such alleles frequently die before have the opportunity to reproduce.

Autosomal Recessive inheritance pattern

These diseases tend to skip generations as they are passed down. Two copies of the abnormal recessive gene are required in order for the diseases or trait to develop.

• tends to skip generations and then reappear.

  • if both parents are carriers = 25% chance of inheriting condition and 50% of being a carrier

Huntington’s Disease

neurodegenerative disorder. caused by mutation in a gene. Is an autosomal dominant disease.

Disease allele is dominant, therefore likely to be passed onto the next generation.
cellular damage to brain which impairs motor control and movements, as well as cognitive and psychiatric decline

• a lethal dominant autosomal inherited disorder that affects the nervous system

• symptoms are not expressed until middle age - after reproductive age.

Phenylketonuria

autosomal recessive disorder caused by a mutation in the gene that codes for an enzyme. → prevents the normal breakdown of a specific amino acid, causing toxic levels to accumulate in the body and brain.

a recessive autosomal inherited disorder that severly affects brain development

recessive diseases within populatioins

the occuerence of severe recessive disorders that are not sex-linked is low. This is because it is highly unlikely that a carrier from one family will mate with another carrier of the same recessive condition.

risk is slightly greater in consanguineous marriages (closely related couples, such as cousins) because there is an increased chance of being a carrier for the same genetic condition.

Sex-linked characteristics

traits that show different patterns of inheritance in the two sexes due to men and women having different sex chromosomes.

red-green colour blindness

sex-linked recessive trait. reduced ability to distinguish between certain colours.

about 8% of males are colour blind (the condition is rare in females)

haemophilia

a sex-linked recessive recessive disorder. blood doesn’t clot normally. when blood can’t clot properly, excessive bleeding (external and internal) occurs after only injury or damage.

hemizygous

term used for males in sex-linked traits because males have no allelic counterpart (i.e. no matching pair)

sex-linked recessive inheritance - women

it is extremely rare for women to acquire a recessive sex-linked trait. Instead females are often carriers

Carriers

one who possess (carries) a recessive allele but does not show the recessive phenotype: a normal dominant allele masks the recessive allele in females.

dominant inhertance pattern in pedigrees

• does not tend to skip generations

• two heterozygous parents with the characteristic can produce offspring without the characteristic

recessive inheritance pattern in pedigrees

• tends to skip generations and then reappear again

• two heterozygous parents without the characteristic can produce offspring with the characteristic.

autosomal inheritance patterns in pedigrees

• each gender is affected in fairly even numbers

• traits can be passed from father to son.

• an affected mother may have an affected or unaffected son

X-linked inheritance patterns in pedigrees

• males (XY) more often, present with the characteristic because they only have one X-chromosome

• trait cannot be passed from father to son (only father to daughter or mother to son)

• Each son of a carrier mother has a 50% chance of being affected

  • All affected mothers will have an affected son.

Co-dominance

when a trait is determined by two equally dominant alleles

polymorphic gene

three or more alternative forms of a gene (allele) for a characteristic.

• although there are many possible options, each person only has two alternatives.

ABO blood groups

Depends on the presence or absence of two different antigens on the red blood cell membrane

A Blood group

IAIA

IAi

B Blood group

IBIB

IBi

AB Blood group

IA IB

O Blood group

ii

antigen

a chemical which has a special property in that it can stimulate the body’s immune system. If the immune system does not recognise the antigen, then an immune response occurs.

IA - red blood cells have the ‘A’ antigen

IB - red blood cells have the ‘B’ antigen

i - red blood cells have no antigens

blood transfusion

the transfer of whole blood or blood components from a healthy donor into the blood stream of a recipient, which requires compatible ABO antigens to prevent agglutination or an adverse immune reaction.

blood transfusion precautions

it is critical than an individual’s red blood cells not come into contact with antibodies against his or her own antigens.

the body has antibodies to foreign antigens. therefore no foreign antigens are allowed when transfusing blood.

can’t give foreign antigens to someone - their blood will clump (agglutination)

agglutination

the clumping together of cells, such as red blood cells, caused by antibodies binding to specific surface antigens, which can occur ruding an incompatible blood transfusion.

AB blood group transfusion

they have both antigens on their red blood cells already, therefore can receive blood from anyone. AB blood group people are universal receivers.

O blood group transfusions

have no antigens on their red blood cells so they can be donated to anyone. universal donors.

they have anti-A and anti-B antibodies in their plasma that would cause the individual’s blood to clump (immune response)