Mendelian genetics pt2 Flashcards
How many chromosomes do we have?
46
How are our chromosomes arranged?
23 homologous pairs
22 pairs of autosomes
& 1 pair of sex chromosomes
What is a Diploid cell?
Contains the diploid number of chromosomes:
2n = 46
What is a haploid cell?
Contains the haploid number of chromosomes:
n = 23
What is mitosis?
Process by which cells are replicated for growth, repair etc
Produces 2 daughter cells each diploid (2n)
What is meiosis?
Process by which gametes are produced
4 daughter cells each haploid (n)
What are the stages of meiosis?
Meiosis 1 –> REDUCTIONAL devision, homologous pairs are seperated reducing chromosomes no. by half
Meiosis 2 –> EQUATIONAL devision, sister chromatids are separated producing 4 haploid gametes
How is meiosis the origins of genetic variation?
- Independent assortment
- Crossing over
- Humans have n=23, no. possible gametes 2^23
How does crossing over cause genetic variation?
- Produces recombinant chromosomes that carry genes from 2 parents
- Avg. 1-3 crossing over events per chromosome pair
- Occurs in prophase 1
How does independent assortment cause genetic variation?
Chromosome placement not influenced by the placement of any of the others
Leads to an individual producing a colllection of gametes that differ from parents
How does random fertilisation allow for genetic variation?
- Random nature of fertilisation = genetic variation
- Fertilisation = 2^23 X 2^23 = 70 trillion diploid combinations
What does non-mendelian patterns of inheritance mean?
Not all traits follow this law:
- Genotypic ratios follow Mendel’s laws but phenotypes do not
- Mendel’s laws do not apply
What is the degree of dominance?
- In Mendel’s pea experiments the offspring always look like one of the parents
- Mendel’s pea alleles show COMPLETE DOMINANCE
What is incomplete dominance?
(A non-Mendelian pattern of inheritance)
- The pheotype of the heterozygote is intermediate between the phenotype of the dominant & recessive traits - blending of the traits
- One trait is not really dominant over the other
e.g. human hair –> curly (homozygous) or straight (homozygous)
What are the non-mendilian patterns of inheritance?
- Incomplete dominance
- Codominance
- Multiple alleles
- Pleitrophy
- Epistasis
- Polyhenic inheritance
- Environmental factors
What is codominance?
(Non-Mendilian patterns of inheritance)
- When BOTH alleles for a trait are expressed in heterozygous offspring
- Two dominant genes are expressed at the same time e.g. MN blood gap glycoproteins on the surface of RBCs
- ABO also co-dominance but 3 alleles rather than 2
What is multiple alleles?
(Non-Mendilian patterns of inheritance)
- Most genes exist as more thna 2 alleles
- e.g. ABO glood group
- 4 phenotypes A B AB O
- IA and IB are co-dominant - both carbohydrates present hwen they exist together in a genotype
What is pleiotropy?
(Non-Mendilian patterns of inheritance)
- Most genes are multiple phenotypic effects (plieotrophy) - not as simple as Mendel’s pea plants
- Pleiotrophic alleles are responsible for the multiple symptoms associated w hereditary disease such as cystic fibrosis & sickle cell anaemia
What is epistasis?
(Non-Mendilian patterns of inheritance)
- Phenotypic expression of gene at one locus affets another gene as a different locus –> e.g. labrador coat
- Black colour dominant to brown BB, Bb, bb= brown
- Second gene determines whether pigment will be deposited in hair - dominant E results in deposition of either black or brown (BbEe) e=blonde
What is polygenic inheritance?
(Non-Mendilian patterns of inheritance)
- One character influenced by many genes
- Generally quantitative traits - continuous variation
- 2 or more genes on a single phenotype, e.g. height or skin colour (the shade you appear)
What are environmental factors in genetic inheritance?
(Non-Mendilian patterns of inheritance)
- In hydrangea flowers;
- Same genetic variety can vary in colour depending on soil pH
What are the instances where Mendel’s laws do not apply
- Mitochondrial inheritance
- Linkage
- Linkage disequilibrium
Why do Mendel’s laws not apply in Mitochondrial inheritance?
mtDNA is solely inherited through the maternal line
Why do Mendel’s laws not apply with linkage?
Two genes that are close together phycially are linked - his laws can’t apply
