1Alterations to Mendel

• Incomplete or partial dominance

• Codominance

• Multiple alleles and Lethal alleles

• Gene interactions & multiple genes– Epistasis and complementation

• Effect of environment

• Extranuclear inheritance

• Sex-linked, sex-limited, & sex-influenced

• Sex determination and Gene dosage

• Polygenics

2The sex chromosomes

• The rest of the chromosomes are autosomes.– Sex chromosomes determine sex of individual.

• X and Y differ in size, X much bigger in humans– But still synapse during meiosis, so still a pair

• In humans, fruit flies, XX = female; XY = male.• In several other organisms, other combinations

of sex chromosomes determine sex.• Because there are genes on sex chromosomes,

inheritance of certain traits can be sex-linked.

3Sex inheritance

• In humans, fruit flies, XX = female; XY = male.

• Inheritance of sex is just like any other trait, except it involve inheritance of an entire chromosome.

Because there are genes on sex chromosomes, inheritance of certain traits can be sex-linked.

Sex Linkage and Determination

• Early 1900s, Thomas Hunt Morgan was doing classical genetics on fruit flies, looking for mutants and checking out the patterns of inheritance.

• He studied the white eye phenotype and discovered something odd…

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5What Morgan saw

Reciprocal cross produced a different result:

Inheritance of eye color depended on sex of the fly.

6Morgan and the F2 generation

When the all red eyed F1 heterozygotes were crossed, close to a 3:1 ratio was observed, but the traits were not evenly divided between the sexes.

7Explanation

• The traits of sex and eye color did not assort independently (as the traits in peas did).

• The traits are linked.• The gene is NOT PRESENT on the Y

chromosome.

R = red eye

R = white eye

XX = female

XY = male

8significance

• With regard to X linked traits, males have only one allele, not two. They are said to be hemizygous.

• Morgan’s work led to the understanding that genes are located on chromosome’s because inheritance of certain traits corresponded to inheritance of a visibly different chromosome.

• Inheritance of X-linked traits results in typical crisscross inheritance: mother to son.

9Crisscross inheritance

http://www.udl.es/usuaris/e4650869/docencia/segoncicle/genclin98/temes_teoria/imatges_temes_teoria/image3.gif

Carrier mother passes allele to son who expresses it, passes allele to daughter who carries it, etc.

Hemophilia & color blindness: examples in humans.

10Sex linked, limited, & influenced

• Sex linked inheritance is when the allele is present on a sex chromosome (usually X).

• Sex limited: when other genetic factors restrict expression to one sex– Bulls don’t give milk.

• Sex influenced: other genetic factors modulate expression. Example: pattern baldness– Females must be homozygous recessive for trait– Even then, hormonal factors

restrict expression.

http://rwqp.rutgers.edu/images/dairy%20cow.jpg

imsc.usc.edu/.../contemporary_sb_3.html

11Sex determination

• Different organisms have different chromosomal mechanisms for determining sex. – XX/XO: typically, the male has one copy.

• Nematodes, e.g. C. elegans – XX/XY: as in humans, fruit flies, XX = female; XY =

male.

• Heterogametic sex is the one that produces a mixture of gametes. Usually the male but:– Female can be heterogametic in some species– Designation is ZZ/ZW where female is ZW

12More on sex determination

• Temperature affects sex determination in many reptile species– Females result from low, high, or extremes of

temperature.– Hypothesis:

13History

• Sex determination studies began in late 1800’s• Work in humans started around 1912, but didn’t

get it right until 1956.• Keys to understanding sex determination in

humans:– Improved karyotype methods– Study of aneuploidy of sex chromosomes– Aneuploidy is the wrong number of a particular

chromosome.– Aneuploidy results from non-disjunction

14Abnormalities in chromosome number result from non-disjunction

Homologues fail to separate during Meiosis I.

15Abnormalities in chromosome number result from non-disjunction-2

Sister chromatids fail to separate during Meiosis II.

16Evidence for XX/XY• 47, XXY Klinefelter syndrome

– male in appearance, but some feminization; sterile.– slow to learn, but not retarded.– XXXY etc. similar, but more severe symptoms

• 45, XO Turner syndrome– Monosomy, the only one occurring in humans– female, sterile, short webbed neck, broad chest,

short.– majority aren’t born

• If XXY is still male and XO is female– Y must be determinant of maleness

17About the Y

• Y chromosome has been shrinking.– Now missing many of genes that X has.

• Two regions: PAR and MSY• PAR= pseudoautosomal region

– Regions near p telomere and q telomere are homologous to X chromosome. Crossing over can occur there during meiosis. Because of this, genes in this location do not behave as sex-linked traits, thus said to be pseudoautosomal because they behave like genes on autosomes rather than sex chromosomes.

Structure of Y18

universe-review.ca/R11-14-Ychromosome.htm

http://www.asiaandro.com/1008-682X/4/259fig.jpg

Human Genome project has revealed much about the Y chromosome.

19Male specific region Y (MSY)

• X-transposed region– 99% identical to X chromosome region, but only 2

genes; the rest are not expressed.

• X-degenerative region– Contains DNA related to X chromosome regions– Several functional genes and pseudogenes– Contains SRY that codes for testis-determining

factor, necessary for maleness during development.

• Ampliconic region– Highly similar or repeated genes, some related to

male development and fertility.

20Evidence for SRY

• SRY contains gene for testis determining factor

• Crossing over in meiosis– Males with two X chromosomes; SRY found on one– Females with X and Y; SRY is missing from Y

• Transgenic mice– Remove SRY from Y chromosome– Mice are XY but are female– Reciprocal experiment also done

Article about Y chromosome

• http://images.google.com/imgres?imgurl=http://www.txtwriter.com/onscience/OSpictures/Y%2520chromosome%2520repair.jpg&imgrefurl=http://www.txtwriter.com/onscience/Articles/ychromosome.html&h=927&w=504&sz=160&hl=en&start=14&tbnid=hTINd2RIkH59cM:&tbnh=147&tbnw=80&prev=/images%3Fq%3DY%2Bchromosome%26svnum%3D10%26hl%3Den%26rls%3DGGLG,GGLG:2005-29,GGLG:en

• Copy and paste

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