Beyond Medelian Genetics

Pedigree Charts Multiple Alleles Incomplete Dominance Co-dominance

PEDIGREE CHARTSPedigree charts show a record of the family of an individual. It can

be used to study the transmission of a hereditary condition. It is particularly useful when there are large families and a good

family record over several generations.

Symbols used in pedigree charts

In a marriage with five children, two daughters and three sons. The second son is affected by the condition.

Organizing the pedigree chartBelow is a pedigree chart of a family showing four generations. A

total of 20 individuals.Generations are identified by Roman numerals. Individuals in each

generation are identified by Arabic numerals numbered from the left. Therefore the affected individuals are II3, IV2 and IV3.

Multiple Alleles

Multiple alleles are different forms of the same gene...that is the sequence of the bases is slightly different in the genes located in the same place on the chromosome.

Although an individual organism can have only two different alleles for a characteristic, when multiple allele forms exist there will be a hierarchy of dominance with each allele is identified with a capitol letter and superscripts of letters or numbers.

For instance, if mice color can be black, yellow, or agouti (combination of colors), and inheritance is due to multiple alleles, what is the phenotype of the offspring from a cross of heterozygous yellow to a heterozygous black

Incomplete Dominance

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

R = allele for red flowers W = allele for white flowers red x white ---> pink RR x WW ---> 100% RW

Co-Dominance

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. 

R = allele for red flowers W = allele for white flowers red x white ---> red & white

spotted RR x WW ---> 100% RW

The ABO blood systemThis is a controlled by a tri-allelic gene. It can generate 6 genotypes.The alleles control the production of antigens on the surface of the red blood cells. Two of the alleles are co-dominant to one another and both are dominant over the third.

•Allele IA produces antigen A•Allele IB produces antigen B•Allele i produces no antigen.

Genotypes Phenotypes(blood types)

IA IA A

IA IB AB

IAi A

IB IB B

IBi B

ii O

Chromosomal Theory

The chromosomal theory of inheritance: Chromosomes carry genes, the units of

heredity. Paired chromosomes segregate during

meiosis. Each sex cell or gamete has half the number of chromosomes found in the somatic cells. (This explains why each gamete has only one of each of the paired alleles)

Morgan’s Experiments

Thomas Morgan (1866 – 1945) conducted experiments on fruit flies

He discovered a number of obvious mutations (a heritable change in the molecular structure of DNA)

Morgan noted a white-eyed male mutation (red-eye is normal)

Mated white-eyed male with red-eyed female and F1 generation was all red-eyed

He then crossed two from F1 generation and F2 generation was ¾ red-eyed and ¼ white-eyed

On examination Morgan discovered that all white-eyed flies were male and that half of the males were white-eyed and half the males were red-eyed

all females were red-eyed Examination of chromosomes in cells showed

that females had four homologous pairs while males had only three plus two non-homologous chromosomes

Sex-Linked Traits

Morgan observed that females has two homologous sex chromosomes (XX) and males had two non-homologous sex chromosome (XY)

From this Morgan concluded that the Y chromosome did not carry the gene for eye colour

Traits located on sex chromosomes are called sex-linked traits

Sex-Linked Traits and Males

Sex-linked traits are controlled by genes located on the sex chromosomes

A recessive trait located on the X chromosome is more likely to be expressed in males than in females since males need only one copy of the recessive allele while females need two

Recessive lethal X-linked disorders in humans occur more frequently in males

Barr Bodies Dr. Murray Barr working at the Univ. of

Western Ontario identified a dark spot in some somatic cells of female mammals

These dark spots are the results of one of the X chromosomes randomly becoming inactive in each cell

These spots are called a Barr body The randomness of inactivation can result in

different cells expressing alternate forms of a gene if the individual is heterozygous