Beyond Mendel1.Thomas Hunt Morgan

(1866 - 1945) and Sex linkage

2.Co-Dominance

3.Environmental Influence

•Gregor Mendel provided for the Scientific community a base upon which to build a new branch of Biology – namely Genetics. His elegant and meticulous experimentation breeding common peas provided a

standard law on how alleles behave during reproduction, both at meiosis when the alleles segregate, and at fertilization when they

come together to determine the phenotype of the organism.•An Englishman, William Bateson (who was a great advocate of

Mendel’s work) started much of genetics culture. He announced 1906 that the 3rd International conference on Hybridization was actually the 3rd International Conference on Genetics (even though there had not

been a second or first!!) and hence a new field was born.•New terms introduced by Bateson were, ‘zygote’ from the Greek zugotos meaning ‘yolked’ to describe the fertilized egg; homo &

heterozygote and ‘allelomorph’ from allelon (of another) and morph (form) to describe different versions of a particular trait. This turned

the new field of genetics into a coherent discipline. •Mendel became the emotional center – easy to be a hero as little was known about him, and what was known was admirable in the sense he

was disciplined, modest, humorous, hard working and humble.•1909 Wilhelm Johannsen (prof of Plant physiology) at Copenhagen

University in Denmark coined the word ‘gene’. Also introduced genotype – an organisms genetic makeup, and phenotype – an

organisms appearance. Nowadays a phenotype is best described as the expression of a gene, as not all traits can be ‘seen’ with the naked

eye and some may develop until later in life.

•There was a combination of reasons why Mendel was not recognised for his work within his own lifetime and one of those was that not all traits behaved in the classic manner that Mendel

described. Many traits did, for example egg shell colour in poultry, wool/coat colour in sheep, mice and rabbits, and various seed

qualities in maize and wheat.•In contrast, there were many more traits that did not follow classical Mendelian inheritance patterns and two examples

studied fell under the category of sex linkage and incomplete or co-dominance.

•I for one was not initially convinced of Mendel’s Laws and based many of my early experiments on these inconsistencies. In 1907 when I was working at Columbia University, I first met Bateson when he came to the US to speak at a Genetics conference. By

then I was beginning to piece together my understanding of Mendelian Genetics, although this did not stop Bateson from

treating my like his Scientific enemy. He accused me of deliberately trying to prove Mendel wrong. We were both

outspoken characters and he took it upon himself to defend Mendel at all costs. Often in a new science, studying what

appears to be a contradiction helps shine a light on the theory being proposed as it uncovers another version. My informative

and significant studies did just that.

•Like Mendel, I focused on one organism, the fruit fly or vinegar fly Drosophila melanogaster. They were quick and easy to breed and

didn’t take up much space. I used empty milk bottles to breed them in and fed them overripe bananas. Males and females were easy to

tell apart and only weeks after fertilization females produced hundreds of eggs.

•Drosophila usually have red eyes – this is called the ‘wild’ type characteristic and is kind of like the default characteristic.

•One day in 1910 I saw a white eyed male and realized I had something important. I kept the fly in a bottle for a few days and took it everywhere with me (some history books even state I took the white eyed fly to the hospital to visit my wife when she was having a baby. It has been told that my wife asked about the fly

before I asked about baby!!!) •I crossed the white eyed male with a pure breeding red eyed

female and in the F1 generation got 100% red eyed progeny. This was consistent with Mendel’s statistics. Red eye was clearly

dominant. •When I crossed this F1 generation they produced the typical

monohybrid Mendelian ratio of 3:1 red to white in the F2 generation. If I recalculated the statistics according to sex, I found that all the females had red eyes and only half the males had red

eyes (the other half had white eyes). So only males had white eyes. This was the beginning of sex linkage in genes.

http://pro.corbis.com/search/searchFrame.asp

Here are the Punnett Squares for the F1 and F2 generations

F1

X X

X

Y

XX

XX

XY

XY100% Red

eyes

F2

X X

X

Y

XX

XX

XY

XYFemale: 100% red eyes/Male:

50% red eyes & 50% White eyes

I concluded that the gene for eye colour was on the X chromosome and there was no corresponding locus on the Y chromosome.

Drosophila only have 4 pairs of chromosomes anyway – 3 pairs of autosomes and a pair of sex chromosomes. Sex linked genes can be on the X or the Y chromosome. Would it be possible to breed a

white eyed female? If so how?

Parents: Red eye female (XX) x White eyed male (XY)

Parents: Red eye female (XX) x Red eyed male (XY)

Gametes: X X & X Y Gametes: X X & X Y

This is how a sex linked trait is conventionally written

X Xr

YXRXR

XrXR XRXr XRY XRYX

XRXR XrXR XRY Xr

Y

F1

100% Red eyes

Red eye

White eye

F2

3:1 Red eyes to white eyes

Pedigree of the Drosophila experiment

X

XF1

F2

Red eyed female

White eyed male

100% Red eyed female

100% Red eyed male

All females red eyed, 50% homozygous for red eye

and 50% heterozygous for red eye

50 % of males white

eyed

50 % of males red

eyed

3:1 Mendelian ratio of red eye to white

eye in F2 generation

Males cannot be heterozygous for red eye as they only have one X chromosome. If they have the white eye allele then they have to

show a white eye phenotype.

Not long after I discovered sex linkage I set out to find more X linked genes and then created a map for chromosome X. When a locus for a gene lies on the X chromosome, the disease is said to

be X-linked. The inheritance pattern for X-linked inheritance differs from autosomal inheritance only because the X chromosome has

no homologous chromosome in the male, the male has an X and a Y chromosome. Very few genes have been discovered on the Y

chromosome. In humans X linked genes include colour-blindness, muscular dystrophy and haemophilia.

http://www.people.virginia.edu/~rjh9u/xped.html

•As with any X-linked trait, the disease is never passed from father to son.•Males are much more likely to be affected than females. If affected males cannot reproduce, only males will be affected.•All affected males in a family are related through their mothers.•Trait or disease is typically passed from an affected grandfather, through his carrier daughters, to half of his grandsons.

http://www.uic.edu/classes/bms/bms655/lesson6.html

X-Linked recessive – two copies of the gene is required for expression in the phenotype

•The trait is never passed from father to son.•All daughters of an affected male and a normal female are affected. All sons of an affected male and a normal female are normal.•Matings of affected females and normal males produce 1/2 the sons affected and 1/2 the daughters affected.•Males are usually more severely affected than females. The trait may be lethal in males.•In the general population, females are more likely to be affected than males, even if the disease is not lethal in males.

http://www.uic.edu/classes/bms/bms655/lesson6.html

X-Linked Dominance – only one copy of the gene is required for expression in the phenotype

•As well as sex linked genes, co-dominance is another variation from Mendel’s classic simple dominance inheritance.

•I didn’t study co-dominance but when this takes place, the heterozygous geneotype shares the effect of its alleles so that they are both expressed in the phenotype. The heterozygote

therefore has a different phenotype to both homozygotes.•In some breeds of cattle, if you cross a white (WW) coat with a

red (RR) coat, all the offspring with be roan (WR). Instead of being only white or red in colour, the coat is a mixture of both colors,

and gives a patched appearance.•In humans the ABO blood group shows co-dominant inheritance.

F1

R R

W

W

RW RW

RW RW

F2

R W

R

W

RR RW

RW

WW

Here are the crosses written out fully for the F1 and F2 generations

100% RW/Roan (note all capital letters are used to designate shared dominance)

25% RR/Red; 50% RW/Roan; 25% WW/White

Parents: RR (Red) x WW (white)

Gametes: R R & W W

Parents: RW (roan) x RW (roan)

Gametes: R W & R W

•As well as sex linked genes, incomplete dominance is another variation from Mendel’s classic simple dominance inheritance.

•I didn’t study incomplete dominance but when this takes place, the heterozygous geneotype shares the effect of its alleles so that

they are both expressed in the phenotype. The heterozygote therefore has a different phenotype to both homozygotes.

•In snapdragons if you cross a red (RR) petal with a white (rr) petal, all the offspring will have pink petals (Rr). Instead of being

white or red in colour, the phenotype of the petal is a blend of both colours, which actually produces another colour.

http://www.people.virginia.edu/~rjh9u/snapdragon.html

•It is not only the genotype that has an effect on the phenotype. The environment can also have an effect on the appearance of an

individual.•Identical twins don’t always look identical as the environment will

have some effect on how they look.•One classic example is the hydrangea. Depending on the pH of

the soil, the flowers change the pigments in their petal and display different colours. If the soil is acidic then the flower appears blue and when the soil is alkaline then the flower is pink. The flowers contain a natural pH indicator called anthocyanin which reacts

with the pH in the soil.

http://pro.corbis.com/search/searchFrame.asp

• Aubusson, P. and Kennedy, E. (2000) Biology in Context. The Spectrum of Life Oxford University Press, Melbourne, Australia.

• Board of Studies (2002) STAGE 6 SYLLABUS Biology Board of Studies, NSW, Australia.

• Husky, Robert (1999) Flower Colour in Snapdragon. Retrieved from the site http://www.people.virginia.edu/~rjh9u/snapdragon.html February 2004.

• Humphreys, Kerri (2003) Biology. Blueprint of Life. Science Press, Australia.• Huskey, Robert (2000) Pedigree of an X-Linked Recessive Trait –

Colorblindness. Retrieved from the site http://www.people.virginia.edu/~rjh9u/xped.html February 2004.

• Kinnear, J and Martin, M (2001) Biology 2 HSC Course: Jacaranda HSC Science John Wiley & Sons, Australia, Ltd.

• Marantz Henig, Robin (2000) A MONK AND TWO PEAS: The story of Gregor Mendel and the Discovery of Genetics Houghton Mifflin, New York, USA.

• Mudie, K. et.al. (2000) Heinemann Biology Malcom Parsons, National Library of Australia, Australia.

• Olave, Gabriela and Pacheco, Alejandra (no date) Who is Mendel and what is Meiosis? Retrieved from the site http://www.acslp.org/homepages/Seifert/Biology/Mendel.html February 2004.

• Schwartz, Shaul (2004) Corbis. Retrieved from the site http://pro.corbis.com/search/searchFrame.asp February 2004.

• Tissot, Robert & Kaufman, Elliot (2002) University of Illinois at Chicago – Human Genetics. Retrieved from the site http://www.uic.edu/classes/bms/bms655/lesson6.html February 2004