Genptype, Phenotype And Inheritance Flashcards
How many genes does each human have and how many copies of each?
- Each human has 25,000 genes
- Each individual has two copies of each given gene (one maternal and one paternal)
- Each individual has two alleles of a gene
- BUT there are many alleles of a gene within a population!
Define homozygous
Two alleles of a gene are the same
Define heterozygous
Two alleles of a gene are different
Define hemizygous
Only one allele o a gene on the X chromosome (i.e males only)
Define dominant
The allele in a heterozygous which determines the phenotype
Define recessive
The non-dominant allele in a heterozygote, i.e. The allele which does not determine the phenotype in a heterozygote
Define co-dominance
Where neither of 2 alleles are dominant over each other e.g. Allele A and B for blood type
Describe blood type A
Antigens on membrane: A Antibodies in blood: anti B Safe to transfuse to: A, AB Safe to transfuse from: A, O Genotype: IAIA or IAIO
Describe blood type B
Antigens on membrane: B Antibodies in blood: anti A Safe to transfuse to: B, AB Safe to transfuse from: B, O Genotype: IBIB or IBIO
Describe blood type AB
Antigens on membrane: A + B Antibodies in blood: none Safe to transfuse to: AB Safe to transfuse from: A, B, AB, O Genotype: IAIB
Describe blood type O
Antigens on membrane: none Antibodies in blood: anti A anti B Safe to transfuse to: A, B, AB, O Safe to transfuse from: O Genotype: IOIO
Describe pedigree symbols
■ affected male □ male ▌carrier male ● affected female ○ female ◐ carrier female ◆ affected unknown sex ◇ unknown sex Diagonal line from bottom left to top right - decreased Shaded dot inside symbol - usually x-linked recessive carrier
name some inheritance patterns
Autosomal
Sex linked
Mitochondrial
Polygenic
Autosomal recessive
Autosomal dominant
X-linked recessive
X-linked dominant
Y-linked (rare)
Describe autosomal recessive inheritance and give an example of an autosomal dominant disease
- Heterozygotes unaffected
- Males and females equally affected
- Two heterozygotes have 25% chance of having affected offspring
- Two affected (homozygous) individuals will have affected offspring only
- Males and females equally affected
- Disease seem to ‘come out of nowhere’
- Disease can skip generations
- Both parents of affected individual are heterozygous ‘carriers’
- For example: Cystic Fibrosis
Describe autosomal dominant inheritance and give an example of an autosomal dominant disease
- Heterozygotes affected
- Males and females equally affected
- Disease rarely found in homozygous state (assume heterozygous unless told otherwise)
- Every affected individual has 50% chance of having affected offspring
- Every affected individual will have (at least) one affected parent
- Disease cannot skip a generation, i.e. affected individual(s) in every generation
- For example: Huntington’s Disease
Describe X linked recessive inheritance and give an example of an X-linked recessive disease
- Hemizygous males (if their X is affected they are diseased) and homozygous females affected
- Disease more common in males
- Heterozygous female ‘carrier’ has 50% chance of having affected sons
- Affected males cannot give trait to sons (as they only give Y chromosome)
- Males and females unequally affected
- Every affected male will have (at least) a heterozygous ‘carrier’ mother
- Every affected female will have an affected father and a ‘carrier’ mother
- Heterozygous daughters of affected males
- For example: Haemophilia A
Describe X-linked dominant inheritance and give an example of an X-linked dominant disease
- Hemizygous males and heterozygous females affected
- Heterozygous ‘affected’ female has 50% chance of having offspring
- Affected males cannot give trait to sons, but will give it to all their daughters (X is affected - will give it to all daughters)
- For example: Rett syndrome
Describe mitochondrial inheritace
All individuals inherit from their mother since mtDNA is inherited only from the mother
Is inheritance monogenic or polygenic?
Can be polygenic, e.g. Albinism - inherited in a recessive manner
Two genes A1 and A2
2x2 alleles: A1, a1, A2, a2
Therefore 2 albino individuals’ offspring can be normal, e.g. a1a1A2A2 x A1A1a2a2 = a1A1A2a2 (offspring is heterozygous for both)
Define linked genes
- Genes on the same chromosome are said to be ‘linked’
- Genes on different chromosomes are said to be ‘not linked’
- Linked genes do not show independent assortment at meiosis
- Recombination frequency between two linked genes is dependent on the distance between the genes
- Genes close together are ‘tightly linked’ - inherited together more frequently
- Genes far apart on the same chromosome almost behave as unlinked genes
