Biology 520 Mendelian Genetics – Chapter 11

Mendel’s technique – fig and 2 p A. Gregor Mendel’s early experiments with peas

Mendel’s seven traits – fig. 11-3, p. 310

See fig 11-4 Terms to know: Homozygous dominantGenotype Homozygous recessivePhenotype Heterozygous

Two traits at once: Independent assortment

Fig , p. 317 Actual results Two-factor cross

Independent assortment: another example

Some human examples

Polygenic inheritance

Human hair color is determined by many genes genes for production of the brown pigment (melanin) genes that regulate production of the brown pigment genes for assembly of the pigment into pigment granules genes for modification of the pigment (producing red pigment) All of these genes work together to produce the final hair color. Some of these same genes also influence eye color and skin color (hence the common association of light hair, light skin, and blue eyes, vs dark hair, dark skin, and brown eyes). In many animals, there are various alleles of the genes that regulate pigment production, resulting in different patterns of dark and light-colored hair. In humans, there are few, if any, such alleles that create color patterns; most of the alleles of these genes affect the overall quantity of pigment, resulting in darker or lighter hair. Therefore, among the many shades of brown, there are contributions from multiple alleles of multiple genes. In general, however, we can make two statements relating the genotype to phenotype:  The more pigment the hair cells produce, the darker the hair color.  Darker hair-color alleles tend to be dominant over lighter alleles.

Eye color is a complex polygenic trait – at least 15 different genes are involved (American Biology Teacher, May 2014).

What about more complex traits like height?

More examples see chap 14

One of these genes (PAH) has 67 known alleles

Musician Woody Guthrie had Huntington’s disease Although a dominant disease, the phenotype varies: males show symptoms in adolescence if the disease is inherited from the father, but in middle-age if inherited from the mother. This is an epigenetic effect. Huntington's disease is caused by a genetic defect on chromosome #4. The defect causes a part of DNA, called a CAG repeat, to occur many more times than it is supposed to. Normally, this section of DNA is repeated 10 to 35 times. But in persons with Huntington's disease, it is repeated 36 to 120 times. As the gene is passed on from one generation to the next, the number of repeats - called a CAG repeat expansion - tend to get larger. The larger the number of repeats, the greater your chance of developing symptoms at an earlier age. Therefore, as the disease is passed along in families, it becomes evident at younger and younger ages.

Using pedigrees

Mitochondrial DNA codes for 13 proteins. An additional 1000 or so are active in the mitochondria but coded by nuclear DNA (Nature, Jan 16, 2014)

American Biology Teacher, Nov 2013

Sex determination systems Other Inheritance patterns

X-linked trait: white eyes in fruit flies

X-linked traits Hemophilia

Nature, Dec 2014

Color-blindness is also X-linked. See p. 395 Genetic testing of the remains indicate that 1 of the 3 daughters was a carrier 

American Biology Teacher, Nov 2013

Incomplete dominance – fig Both genes are expressed if both are present

Cholesterol commercial

Blood types – multiple alleles and co-dominance See fig 14-5, p. 394; also p. 320 “analyzing data” See also p. 320: Multiple alleles in parakeet color

Blood Type & RhHow Many Have ItFrequency ORh Positive1 person in 337.4% ORh Negative1 person in 156.6% ARh Positive1 person in 335.7% ARh Negative1 person in 166.3% BRh Positive1 person in 128.5% BRh Negative1 person in 671.5% ABRh Positive1 person in 293.4% ABRh Negative1 person in 167.6%

More here:

Sickle-cell: simple gene with many phenotypic effects and an interesting geographic connection p. 398 and 400

Sickle-cell anemia

Normal situation – independent assortment

Crossing over

Gene linkage and independent assortment + gene maps p

A gene map See also fig

Genome organization: gene vs. locus vs. allele The address numbers here are analogous to a gene locus. 173 and 125 would be homozygous, while 211 and 267 are heterozygous. 321 does not represent a pair of alleles.

Fruit fly chromosome map

Epistasis – the presence of one gene affects another

Gene pathways

Epigenetics refers to inherited changes to the structure of the chromosomes or associated proteins, rather than the genetic sequence. One common example is methylation – methyl groups (-CH 3 ) are often added to DNA

Nature, Oct 21, 2010

Summary of inheritance patterns you should know: Autosomal vs. X-linked Dominant vs. recessive Incomplete or Co-dominance Multiple alleles (ABO blood types) Polygenic inheritance (height, skin color, hair color) Linked genes Pleiotropy (one gene influence multiple traits – sickle cell disease) Heterozygote superiority (sickle cell) Dihybrid crosses (2 traits at once)