more genetic and inheritance stuff

Question 1

Codominance

Answer 1

alleles that are both expressed if they are present together in a heterozygous organism e.g. sickle cell anaemia (Hb^A Hb^S – person with sickle cell trait)

o Alleles I^A and I^B are dominant to i (recessive)
o I^A and I^B are codominant

Question 2

Describe the determination of sex in humans – XX and XY chromosomes.

Answer 2

• Inheritance of sex is controlled by sex chromosomes
• Each of us has 1 pair of sex chromosomes (XX – female; XY - male) along with 22 pairs of autosomal chromosomes
• All female gametes carry an X chromosome but half of male gametes carry an X chromosome and half carry a Y chromosome
• The sex of offspring in humans is decided by which type of male gamete fuses with the female gamete – i.e. by the male

Question 3

Incomplete dominance

Answer 3

the phenotype of F1 hybrid is somewhere between the phenotypes of 2 parental varieties
o The appearance of F1 hybrids falls between the phenotypes of the 2 parents

Question 4

Sex linkage

Answer 4

any gene present on the sex chromosomes that determine sex-linked characteristics is likely to be inherited with the sex of the individual

o X chromosome is much longer than the Y chromosome (mostly concerned with male structures and functions) so many of the genes on the X chromosome are absent from the Y chromosome

o For a recessive sex-linked trait to be expressed
 A female needs 2 copies of the allele
 A male needs only 1 copy of the allele
-
o In a male, an allele present on the X chromosome is most likely to be on its own and will be apparent in the phenotype even if it is a recessive allele
o A human female can be homozygous or heterozygous with respect to sex-linked characteristics, whereas males will have only 1 allele

o Sex-linked recessive disorders are much more common in males than in females

Question 5

Discontinuous variation

Answer 5

• Characteristic concerned is 1 of 2 or more discrete types with no intermediate forms e.g. human ABO blood grouping, tall or dwarf, garden pea plant, rolling of tongue
• Controlled by one gene with 2 alleles
• Clear-cut difference in an inherited characteristic with no intermediate form and no overlap between both phenotypes
• Unaffected by environmental conditions/factors –> genetically determined

Question 6

Continuous variation

Answer 6

• Results in a continuous distribution of values e.g. height in human, body weight, skin colour
• Controlled by a number of genes = polygenes –> effect of any one of these genes make a very or insignificant effect on the phenotype, but the combined effect of all the genes of the polygene is to produce infinite variety among the offspring
• No clear-cut difference with infinite varieties
• Affected by environmental conditions/factors –> may be genetically determined or may be due to environmental and genetic factors working together

Question 7

Law of segregation

Answer 7

o During meiosis, pairs of alleles separate so that each cell has 1 allele of a pair
o Allele pairs separate independently during the formation of gametes

Question 8

Law of independent assortment (only applies to genes on different, non-homologous chromosomes)

Answer 8

o During meiosis, the separation of 1 pair of alleles is independent of the separation of another pair of alleles
o The inheritance of 1 character has no effect on the inheritance of another