T8 - Genotype, Phenotype + Inheritance Flashcards
define genotype and phenotype
genotype is the genetic makeup of an individual
phenotype is the observable characteristics of an individual resulting from the interaction of its genotype with the environment
define gene and allele
gene is a regent of DNA that codes for a particular polypeptide which is located on a chromosome
allele are variants of the same gene at the same position on a chromosome
the combination of alleles an individual carries influences their phenotype
homozygous/heterozygous/hemizygous
individuals can be…. for an allele
- HETEROZYGOUS: the two alleles of the gene are the same, and the trait is expressed
- HOMOZYGOUS: the two alleles of the gene are different. A carrier for ‘a’. The dominant trait is expressed in the phenotype.
- HEMIZYGOUS: only one allele of a gene is present, and this is on the X chromosome. Males are XY and therefore may only have one allele of gene on X chromosome
modes of inheritance
- dominant: the dominant allele in a heterozygote determines the phenotype
- recessive: the non-dominant allele in a heterozygote is called recessive. Only expressed in homozygous recessive
- co-dominance: equal influence of two alleles on a phenotype…neither allele can mask the expression of the other
example of autosomal dominant diseases in humans
ie inheriting a single copy of the gene leads to the phenotype being expressed
eg bradydactyly controlled by single dominant gene, IHH
Huntington’s disease caused by Huntingtin gene, abnormal number of polyglutamine repeats. Atrophy of basal ganglia, which is important for movement. Leads to neurodegeneration and choreiform movements.
Autosomal means it’s on the normal chromosomes, not the sex chromosomes. Males and females are equally affected.
example of autosomal recessive inheritance
eg cystic fibrosis
- controlled by CFTR gene
- males and femals equally affected, autosomal
- heterozygotes unaffected
- two heterozygote carriers will have 25% chance of having affected offspring
pedigree tree key
- male is square, female is circle, unknown is diamond
- affected is black, unaffected is white
- carrier is half black, half white
- carrier for X-linked condition is white with black dot inside
- deceased is crossed through
what is X-linked recessive inheritance
males = XY females = XX
X-linked diseases refer to disease for which genes are carried on the X chromosome
- therefore, females must be homozygous recessive in order to express trait
- males only have one X chromosome and therefore need only one recessive allele on X chromosome to express the trait
- therefore males are hemizygous
- males are therefore much more likely to be affected by these diseases
- affected males can’t give trait to their son, but only their daughters
- every affected female must have an affected father and a carrier mother
example of X linked recessive inheritance
haemophilia A
- caused by F8 gene on X chromosome
- reduction of protein called factor VIII leads to blood clotting problems
- males and females unequally affected
- affected males will have (at least) a heterozygous carrier mother
- every affected female will have an affected fatehr and a carrier mother
finish this card
X-linked dominant inheritance and example
fragile X syndrome
- X chromosome is constricted and likely to break
- associated with intellectual disability and autism
- X linked diseases are extremely rare
- hemizygous males and heterozygous females affected
- more commonly seen in males, but in general, exact pattern of inheritance varies for other X-linked dominant diseases
- all fathers that are affect will have affected daughters, but not affected sons
- heterozygous female has 50% chance of having affected offspring
- some conditions are embryonic lethal for one gender, making them to appear more commonly in one gender
another example of disease is incontienta pigmenti - a progressice dermatological disorder affecting the skin, hair, teeth and nervous system
Y-linked diseases
- males are the only gender to have the Y chromosome
- therefore these diseases only affect males
- the Y chromosome contains genes which drive normal male development
- as there is only one Y chromosome in males, the allele cannot be recessive, as it will always have an effect on development
- these are even rarer than X linked diseases
rare so hard to find info about them in medical literature
an example of Y-linked disease is azoospermia so that is the complete inability to produce sperm, leading to infertility
what is mitochondrial inheritance
- mitochondria contain DNA (mtDNA)
- used to encode proteins that are needed for oxidative phosphorylation
- mtDNA is only inherited from mother
- this is because the mitochondria of the egg are used to produce all the mitochondria for the cells of the body
- a mutation in mtDNA will cause malformation of the proteins required to generate ATP for cells
- this will lead to mitochondrial disease
…therefore affected mother will always have affected children
example diease: kearns-sayer syndrome which causes a reduction of heart, eye and muscle movements
what is mitochondrial inheritance
- mitochondria contain DNA (mtDNA)
- used to encode proteins that are needed for oxidative phosphorylation
- mtDNA is only inherited from mother
- this is because the mitochondria of the egg are used to produce all the mitochondria for the cells of the body
- a mutation in mtDNA will cause malformation of the proteins required to generate ATP for cells
- this will lead to mitochondrial disease
…therefore affected mother will always have affected children
example diease: kearns-sayer syndrome which causes a reduction of heart, eye and muscle movements
what is polygenic inheritance
- many phenotypes are influenced by multiple genes
- ie they are not monogenic
- eg obesity, cancer, diabetes, skin colour and height
- if enough genes are involved, the trait becomes continuous (very small graduations between possible phenotypes)
- eg albinism, on seperate card
what are monogenic traits
- influenced by activity of single gene
- monogenic traits are discontinuous
- means can see the absence or presence of a trait but nothing in between
