1 - Mendelian Inheritance and the role of meiosis in determining the pattern of inheritance Flashcards
Who was Gregor Mendel?
Father of genetics
what did Mendel’s work with peas lead to?
- discovery of dominant and recessive traits
- concept of the gene, formulation of the basic laws of inheritance
character
heritable feature of an individual
trait
a variant form of a character (phenotype)
What is the law of segregation?
The 2 forms of a gene (alleles) present in each parent segregate independently
how did Mendel formulate the law of segregation
by studying the results of monohybrid crosses
What is a monohybrid cross?
a cross between two true- breeding individuals differing in only 1 character
What did Mendel observe from monohybrid crosses?
- All F1 progeny resembled one of the parents, but both parental traits appear in the F2 generation
- the ratio in the F2 generation is 3:1
Monohybrid cross conclusions
- 1 trait is dominant and the other is recessive
2. the heritable factor for the recessive trait been masked by the presence of the factor for the dominant trait
What is Mendel’s model?
- Variations in inherited characteristics are due to the existence of alternative versions of alleles
- For each character, an organism inherits 1allele from each parent
- If the 2 alleles differ, then the dominant allele determines the organism’s phenotype
- The alleles remain discrete when present in
- The 2 alleles segregate during gamete formation - and end up in different gametes
How do offspring receive genes?
- Each parent has 2 alleles - but only 1 is passed to an individual offspring via the gamete
- Each offspring receives 1 allele from 1 parent and the other allele from the other parent
How segregation of alleles can explain the 3:1 ratio seen in the F2 generation
1 homozygous dom,
2 heterozygous dom,
1 homozygous rec.
3:1 ratio of phenotypes
What is a punnett square?
A diagrammatic device for predicting the outcomes of crosses between parents of known genotype
what are the 3 possible combinations of 2 alleles?
- Homozygous dominant : GG
- Homozygous recessive : gg
- Heterozygous : Gg
Different genotypes can produce the same phenotype
What is a test cross?
A method for determining the genotype of an individual with the dominant phenotype of a trait
How do we determine the genotype of an organism with a dominant phenotype?
cross the test species with a homozygous recessive individual
What is Mendel’s Law of Independent Assortment?
Each pair of alleles assorts independently of each other pair of alleles during gamete formation
What is a dihybrid cross?
a cross between two true- breeding individuals differing in 2 characters
What is seen in the F2 generation of dihybrid crosses?
- 2 recombinants in addition to the 2 parental phenotypes
- 9:3:3:1 ratio
What would happen if independent assortment did not occur?
- The gametes produced in the F1 would be of only 2 types (YR and yr)
- the F2 would have no recombinant phenotypes - only parentals
explanation of LIA observations
- there are 4 possible combinations in each gamete, not 2.
- F2 progeny ratio - 9:3:3:1
How to deal with a triple heterozygote
e.g RrYyGg x RrYyGg, how many will have round, yellow, seedsd in yellow pods?
deal with each of the 3 allele pairs separately
Meiosis
- results in 4 daughter cells each with half the number of chromosomes of the parent cell
- as in the production of gametes and plant spores.
Mitosis
cell division in which the nucleus divides into nuclei containing the same number of chromosomes
Comparison of Meiosis and Mitosis
- Mitosis occurs in somatic cells, meiosis occurs in the germ line
- In mitosis there is only 1 cell division, in meiosis there are 2
- Meiosis produces 4 (non-identical) haploid cells (gametes), mitosis produces 2 identical diploid cells
- Synapsis is unique to meiosis
- The role of mitosis is to drive growth and tissue repair; the role of meiosis is to produce haploid gametes and to introduce genetic variability
Homologues
The 2 chromosomes of a homologous pair are individual chromosomes that were inherited from different parents
- appear alike under a microscope but have different versions of a gene at some of their loci
Chromatid
one of the two identical strands of a newly replicated chromosome
sister chromatids
2 identical chromatids held together by a common centromere following replication
stages of meiosis I and II
Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II
How did sutton make the link between behaviour of chromosomes during meiosis and medel’s laws?
He observed that:
- Chromosomes occur in pairs in somatic cells
- Chromosome pairs segregate equally into gametes
- Different chromosome pairs assort independently
chromosome theory of inheritance
States that:
- Mendel’s ‘heritable factors’ (or genes) are located at specific positions (loci) on chromosomes
- It is the chromosomes that undergo segregation and independent assortment
How does chromosome behaviour in meiosis explain Mendel’s Law of Segregation?
Because each allele is on a different member of a chromosome pair and moves to opposite poles in anaphase I
How does chromosome behaviour in meiosis explain Mendel’s Law of Independent Assortment?
Because the arrangement of chromosomes at metaphase I is random
