Inheritance

What are homologous chromosomes

• diploid organism has two sets of chromosomes, one from each parent

• chromosomes of a homologous pair:

  • pair with each other during prophase I of meiosis

  • have exactly the same order of gene loci

  • have centromeres in the same position and arms of the same length

• chromosomes which determine the same characteristics (e.g. eye colour, blood group) are called homologous chromosomes (or homologs), they do mot need to be genetically identical

What is a gene

gene is a discrete unit of hereditary information.

it is a specific sequence of nucleotides (Fig. 1.3) in the DNA molecule which codes for a RNA (e.g. tRNA, rRNA, telomerase RNA) or polypeptide (e.g. amylase)

What is a gene locus

position occupied by a gene on a chromosome

What is a allele

alternative form of the same gene responsible for variation in a particular characteristic

alleles of a gene occupy the same gene locus on homologous chromosomes

alleles cause variation due to a slight difference in the nucleic acid sequence, which give rise to different forms of the protein.

What is pure breeding

members of a pure line are homozygous at the gene loci and are said to breed true

What is phenotype

phenotype is the observable characteristics of an individual, this is dependent on the genotype

What is genotype

genotype is the genetic makeup of an organism determined by the alleles present in the genome.

What is dominant allele

an allele that is fully expressed in the phenotype even in the presence of a differentallele, i.e. expressed in both homozygous and heterozygous condition.

What is recessive allele

allele which is expressed in the phenotype only in the presence of another identical,allele, i.e. expressed only in homozygous condition

What are mulitple alleles

• gene that has more than two different types of alleles in a given population

• regardless of the number of different alleles, only two can be present in a diploid organism

What is codominace

phenomenon in which two alleles which are expressed equally in the phenotype of a heterozygote

What is the explanation for co dominance

• four blood groups (A, B, AB and O) is a result of different combinations of 3 alleles IA, IB and IO

• IA codes for an enzyme the converts antigen H to antigen A on red blood cells

• IB codes for another enzyme that converts antigen H to antigen B on red blood cells

• IO codes for a non-functional enzyme. Antigen H remains as it is (blood group O).

• alleles IA and IB are codominant. A heterozygote (IAIB) produces both enzymes, resulting in both antigen A and B on red blood cells

• both alleles IA and IB are dominant to the IO allele

What is incomplete dominace

phenomenon in which one allele for a specific trait is not completely dominant over the other allele, resulting in heterozygote offspring having a phenotype which is intermediate of that of the homozygous parents

What is an example of incomplete dominance

R represents the allele that codes for an enzyme that synthesizes red pigment

r represents the allele that codes for a non-functional enzyme that cannot synthesize any pigment

in RR genotype, there is sufficient enzymes present → large amount of red pigments synthesized → red colour

in Rr genotype, there is insufficient enzymes present → small amount of red pigments synthesized → pink colour

in rr genotype, there are no enzymes → no pigments synthesized → white colour.

State Medel’s first law

each pair of alleles of a gene segregates and each gamete receives one of each pair of alleles

Why did medel choose pea plamt

• there were several varieties available, which had quite distinct characteristics, many traits occur in two alternate forms that are easy to distinguish

• plants were easy to cultivate and produce many offspring.

• short generation time - develop quickly hence results can be seen within a short time.

• reproductive structures were completely enclosed by the petals, which ensure self-pollination, leading to pure breeding, whereby the same characteristics are produced generation after generation

• artificial cross-breeding between varieties was possible, resulting in hybrids that were completely fertile.

What is mono hybrid

inheritance of one characteristic controlled by alleles of one gene

What is dihybrid

dihybrid inheritance involves the inheritance of two characteristics which are controlled by alleles of two different genes, respectively

What is Mendel’s second law,

law of independent assortment or Mendel’s Second Law states that the alleles of one gene segregate independently of the alleles of another gene during gamete formation

State Morgans experiment explanation

• genes for these 2 characteristics are on the same chromosome (i.e. they are linked).

• they are therefore usually inherited together

• the appearance of the non-parental (recombinant) phenotypes is due to crossing over during prophase I of meiosis.

• there will be no fixed ratio for offspring of different phenotypes.

What is a linkage group

All genes on the same chromosome form a linkage group and are usually passed into the same gamete. These genes are inherited together, and as a result, genes on the same chromosome usually do not show independent assortment.

How is genetic recombination formed

• crossing over results in genetic recombination

• gamete that contains the recombinant chromatid is known as recombinant gamete

• offspring that exhibits new combinations of

• characteristics that are different from their parents are known as recombinant

• sequence of events accounts for the occurrence of recombinant phenotypes in Morgan’s test cross

How is sex determined

• microscopic examinations of the chromosome structure of a range of mammals revealed that males and females showed certain chromosomal differences

• homologous chromosomes are found in all cells except for one pair of chromosomes which always shows differences between the sexes

• these are known as the sex chromosomes (heterosomes) sex chromosomes (named X and Y) are involved in sex determination. All other chromosomes are known as autosomal chromosomes or autosomes

• in humans, for example, the first 22 pairs of chromosomes are autosomes. In human females, the 23rd pair is a pair of sex chromosomes which are homologous. In human males, however, the sex chromosomes are not homologous

What is the female genotype in humans concerning sex chromosomes?

• two X chromosomes (genotype XX).

• during gamete formation, the female only produces one type of gamete (with respect to sex chromosomes) – all the egg cells contain an X chromosome.

• humans, the female is the homogametic sex.

Describe sex link

• genes carried on the sex chromosomes are said to be sex-linked

• Y chromosome is smaller than the X chromosome, and carries very few genes compared to the X chromosome, which carries over 1,000 genes. There is a portion of the X chromosome with no homologous region on the Y chromosome

• characteristics that are determined by genes carried on the non-homologous region of the X

• chromosome will appear in males even if they are recessive

• males have a higher tendency to inherit sex-linked diseases as they only have one X chromosome, whereas females have two X chromosomes

What is the male genotype in humans concerning sex chromosomes?

• one X chromosome and one Y chromosome. (Genotype XY).

• during gamete formation, the male produces two types of gametes (with respect to the sex chromosomes) - 50% of the sperms carry the X chromosome and the other 50% carry the Y chromosome

• humans, the male is the heterogametic sex

Why is it more common for males to express X linked traits

• males carry only one X chromosomes

• thus they cannot be heterozygous or carrier for colour vision gene

• only one copy of X chromosome liked allele is sufficient to cause them to be colour blind

State autosomal dominant inheritance pattern

• trait is determined by a dominant allele

  • every affected individual has at least an affected parent

  • it appears in every generation

• trait is autosomal (i.e. not sex-linked)

  • equal numbers of males and females are affected

  • male lpasses the trait to his son. In sex-linked, father contributes the Y-chromosome to his son hence does not pass on the X-linked disease to his son

State autosomal recessive inheritance pattern

• trait is determined by a recessive allele

• an affected individual may have unaffected parents

• the recessive trait skipped a generation

• trait is autosomal (i.e. not sex-linked) because

• unaffected heterozygote parents each passed a copy of the recessive allele to the affected daughter

• if the recessive allele is sex-linked, the daughter would not be affected as she would inherit the dominant allele on the X chromosome (normal) from the unaffected father

State sex linked recessive inheritance pattern

This trait is determined by a sex-linked recessive allele because

o none of the sons of an affected male (and an unaffected female) is affected

o sons of an affected female are affected (not shown in Fig. 5.5)

o more males than females are affected

State sex linked dominant inheritance pattern

• trait is determined by a sex-linked dominant allele

• all daughters of an affected male individual are affected

• affected male does not pass the disease to his sons, since sons receive Y chromosome from their father.

How does crossing over in meiosis I increase variation

• homologous chromosomes pair up

• chiasmata form between non-sister chromatids of homologous chromosomes.

• corresponding non-sister chromatids may break and rejoin at any point along their length where chiasmata formed

• as a result, corresponding alleles are exchanged

• chiasmata can occur almost anywhere on the chromatids, and the number of chiasmata may vary from zero to as many as eight

• amount of genetic variation resulting from this reshuffling of alleles between homologous chromosomes can be infinite.

• produces new linkage groups and so provides a major source of genetic recombination of alleles

• exact genotypes produced depend on the number of chiasmata and the positions of the genes relative to the sites of crossing over

• crossing over is not the same as mutation as crossing over does not create new genes / alleles. It merely reshuffles the alleles.

How does random fertilisation help variation

• random nature of fertilization adds to the genetic variations arising from crossing over and independent assortment and segregation of chromosomes in meiosis

• zygote is formed when an ovum (1 of approximately 8 million) is fertilized by a single sperm cell (1 of approximately 8 million possible combinations).

• without considering crossing over, random fusion of sperm and egg will produce a zygote with any of the over 70 trillion diploid combinations (223 x 223).

• if crossing over is considered, the number of possibilities is infinite

How does Independent assortment and segregation of chromatids during metaphase II and anaphase II of meiosis respectively help variation

• during metaphase II, the orientation of pairs of chromatids is also random and determines which

• chromatids migrate to opposite poles of the cell during anaphase II.

• gives rise to a large number of different chromosome combinations in the gametes.

• independent assortment segregation alone, the number of possible combinations of gametes during meiosis is 2n, where n is the haploid number of the organism.

• humans, the number of possible combinations is 2²³ (about 8 million).

• together with crossing over, the number is far more than 2²³

How does Independent assortment and segregation of chromatids during metaphase I and anaphase I of meiosis respectively help variation

• orientation of the homologous chromosomes on the equatorial spindle during metaphase I of meiosis determines the direction in which the homologous chromosomes separate during anaphase I.

• orientation of the homologous chromosomes is random.

What is the difference between discontinuous and continuous variation (no of genes)

discontinuous variation: determined by one or a few genes and the effects of each gene is discernible, different alleles at a single gene locus have large effects different gene loci have quite different effects on the phenotype.

continuous variation: determined by many genes, these different gene loci have same and often additive effect on phenotype, different alleles at a gene locus has a small effects, thus phenotypic expression is the result of all these genes

What is the difference between discontinuous and continuous variation (type of phenotypic classes)

discontinuous variation: discrete groups

continuous variation: a range of values between 2 extremes

What is the difference between discontinuous and continuous variation (inheritance)

discontinuous variation: qualitive

continuous variation: quantitive

What is the difference between discontinuous and continuous variation (method of measurement)

discontinuous variation: recognition of different qualities and summation of all the traits

continuous variation: recognising of quantity via measurement

What is the difference between discontinuous and continuous variation (effects of environment)

discontinuous variation: no effect

continuous variation: affect by environment