Inheritance Chapter 29

Gregor Mendal “Father of Genetics”

What Mendal did  He bred peas in the monastery garden at Brno, Czech Republic (then part of the AustroHungarian Empire).  Observed occasional variations in the appearance of these plants.  Selectively bred plants to consistently produce “characteristics” that were unusual.  Saw a pattern in the way that the unusual characteristics showed up.  Was the first to propose that these characteristics were passed from one generation to another by the gametes.

The Abby where Mendal worked

What Mendal did not do  He didn’t use the word “gene” to refer to subject of his work.  He didn’t see chromosomes.  He never used a Punnett square.  He never achieved fame in his lifetime for his work.

Charles Darwin Proposed the “Theory of Evolution”. Actually, talked about “descent with modification from a common ancestor”. He didn’t use the word “evolution” very often. Voyage of the Beagle 1831 – Presented paper with Alfred Russell Wallace in Published first edition of “Origin of Species” in 1859.

Some Vocabulary  Genetics – study of inheritance.  Autosomes – the 22 pairs of chromosomes that do not determine genetic sex.  Sex chromosomes – the 23 rd pair, the X and the Y.  Karyotype – the diploid chromosomes displayed in their condensed form and paired as homologs

A typical karyotype

More Vocabulary  Alleles - a matched pair of two genes, coding for the same or alternative forms of a particular trait. Found at the same location (locus) on homologous chromosomes.  Homozygous – having the same alleles for a trait  Heterozygous – having different alleles for the same trait.

More words  Dominant – an allele that expresses itself and masks its partner. Example: brown hair is dominant over blond.  Recessive – the reverse of the above. The allele that is masked  Allele pairs are expressed as a pair of letters representing the trait. Example: Mendal’s peas came in tall and short. Tall is the dominant allele for height in peas. Therefore it is written as a capital “T”.  A heterozyote for height would be Tt, with the lowercase t representing the recessive.

Genotype vs. Phenotype  Genotype – the actual alleles an organism has is it’s genotype. In our heterozygote pea plant that would be Tt.  Phenotype – that which is expressed. Our pea plant maybe genotypically heterozygotic but phenotypically it is tall.  Homozygote dominant = TT phenotype = tall  Homozygote recessive = tt phenotype = short  Heterozygote= Tt phenotype = tall

Sources of variation: segregation & independent assortment Assortment leads to many possibilities as far as gamete formation goes. For any genome it can be calculated as 2 n, where n=the number of chromosome pairs.

So for a human with 23 chromosome pairs, the possible combinations of gametes = 2 23 or 8,388,608! (and that’s with out recombination)

Mendal’s Laws  Mendal discovered that if you bred plants that had two alleles for each trait that you would get the same ratios of phenotypes & genotypes whenever you crossed heterozygotes. It was like clockwork!  This was because of independent assortment and segregation, which became known as “Mendal’s Laws”

It works like this… Phenotypic ratio = 3:1 or 3 tall : 1 short Genotypic ratio = 1:2:1 or 1 homozygote dominant 2 heterozygotes 1 homozygote recessive

Example: PKU

Violation of Mendal’s Laws  Mendal’s laws only hold if: there is random fertilization there is random fertilization the alleles are located on separate chromosomes the alleles are located on separate chromosomes the alleles have a simple dominant/recessive relationship the alleles have a simple dominant/recessive relationship there are only two alleles for that trait there are only two alleles for that trait they are not lethal to the zygote they are not lethal to the zygote

Recombination interferes with Mendal’s laws

Types of inheritance Aside from simple dominant/recessive  Incomplete dominance – a dominant allele does not completely mask the recessive (red flower + white flower = pink flower).  Codominance – both traits are expressed together (red flower + white flower = stripes).  Multiple alleles – More than one allele for a trait. ABO blood group is an example.  Polygene – several alleles interact to produce a trait. Results are a continuous or quantitative phenotype, as in skin color.

Incomplete dominance: Sickle Cell

Codominance of multiple alleles

Polygenic inheritance

Sex-linked inheritance  Males only have one X chromosome. Therefore, if a trait is found only on the X it will be expressed in a male regardless of whether it is dominant or recessive.  X – inactivation occurs in females. Every normal woman has two Xs but they only need one. Therefore, one X chromosome turns off, forming a Barr body.  Because X – inactivation is random in most cases, it leads to a fine mosaic of cells in females.

22 autosomes & 1 set of sex chromosomes

Sex determination in humans

Color- blindness: a sex-linked trait

Environmental influences  Phenocopy – Developmental influences impact genetic expression in ways that appear to be genetic but are not inheritable.  Temperature, nutrition, non-genetic pathologies can have impacts that are expressed in ways that appear genetic.

Genetic defects  Aneuploidy – a defective set of genes.  Triploidy – an extra set of chromosomes  Trisomy – an extra single chromosome  Monosomy – a missing homolog  Trisomy of the 23 rd chromosome – XXX = “super female” XXY = Klinefelter’s syndrome  Trisomy of the 21 st chromosome leads to Down’s Syndrome.

Down’s syndrome

Klinefelter’s - a type trisomy affecting the sex chromosomes

Turner’s Syndrome: monosomy of the 23 rd chromosome, X_

Monosomy of the 23rd chromosome Name that condition!

A Pedigree: tracking genetic traits

A Peek into the Future: Screening for genetic disorders

That’s all folks!