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1- Morgan and Drosophila
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2- Sex determination and chromosomes
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3- Sex linked traits
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4- Sex linked human disorders
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5- Autosomal dominant human disorders
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6- Autosomal recessive human disorders

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Fruit flies have only 4 chromosomes
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Easy to store
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14 day life cycle

Sex Determination and Chromosomes
Each human cell contains 46 chromosomes:
22 pairs of autosomes
1 pair of sex chromosomes
Sex chromosomes are designated as:
“X” and “Y”
XX= female
XY= male

The sex of a human is genetically determined at fertilization when a sperm cell containing either the X or Y chromosome unites with an egg cell containing the X chromosome.

Sex Determination

Female and Male Sex Chromosomes
X X s c h
S
C
H s
X
Y

Who gives the sons the eye color gene? Mom or Dad?

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Morgans work with Drosophila demonstrated that genes for certain traits are located on the X chromosome and do not appear on the Y chromosome.
Genes found on the linked genes
X chromosome are said to be sex-

Recessive traits that are sex-linked occur more frequently in males than in females.
For the recessive trait to show in a female, she must be homozygous recessive.
The gene must be present on both of her
X chromosomes( X X).

1.
2.
Hemophilia- disease in which the blood does not clot properly.
Colorblindness- inability to see certain colors, most commonly red and green.
* Both of these disorders are more common in males than in females because a female will not show the disorder as long as she has one normal gene. Females who are heterozygous for a sex-linked trait are said to be carriers for that trait.

Color blind punnett square
C = normal color vision c = color blind

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Which child is colorblind? Explain in terms of the alleles he or she inherits.
Which child is a carrier? Explain in terms of the alleles he or she inherits.
If the father had been red-green colorblind and the mother was still a carrier , which children would be colorblind? Which children would be carriers? Can a son be a carrier? Create a Punnett square to find out.

H = normal h = hemophilia hemophilia

What do you think?
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Can a father with hemophilia pass the disease on to his son if the mother is homozygous for the normal allele?
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What will the genotype of the daughters be?
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What percentage of the daughters will be carriers of the disease?

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Normally two working copies of every gene in each individual .
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In dominant genetic disease, one copy of the gene is altered by mutation and causes the disease
 achondroplasia (a form of dwarfism), neurofibromatosis, and Huntington disease .

Try some examples
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(3) Joan and John both have achondroplasia.
They are trying to decide whether to have children. What is the probability that any fetus they produce will be homozygous dominant for achondroplasia and suffer skeletal abnormalities causing a spontaneous abortion?
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25%
A
A
AA a
Aa a
Aa aa

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(4) Zach's mother has achondroplasia but Zach does not. What is the probability of Zach passing achondroplasia to his children?
0%
(5) Margaret, who is normally statured is married to a man who has achondroplasia. What is the probability that their first child will have achondroplasia?
50%

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 parent who shows the trait will pass the mutation on to half of his/her children with an equal chance for sons and daughters to be affected.
Children who do not have the trait will generally not pass the disease on to their children. observed in each generation, usually without skipping a generation.

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 both parents must be carriers (i.e., they are clinically normal but have one mutation of a particular gene) both must pass the mutation to a child in order for that child to be affected.
This inheritance pattern is distinctive in that the parents and other relatives of the person with the disease appear to be completely normal, while 25% of their brothers and sisters will share the same disease

 mutation travels unobserved (silently) within the family and is expressed by siblings in a single generation
 sickle cell anemia , cystic fibrosis ,
Tay-Sachs disease , and phenylketonuria .

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Every organism has thousands of genes
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Genes that are on the same chromosomes make up a linkage group
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Humans have 23 linkage groups
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Linked genes do not separate independently
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Linked genes will not follow the dihybrid ratio of 9:3:3:1
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What’s going on here?

Crossing Over (not this kind  )
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Exchange of pieces of homologous chromosomes
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Occurs during synapsis of the first meiotic division when chromotids are in close contact
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Distant genes are separated more often than closely linked
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Creates new gene linkages
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Thus new variations (think genetic diversity and evolution!)

bbcc x BBCC becomes bbcC x BBcC

Multiple Gene Inheritance (Polygenic)
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When 2 or more independent genes affect one characteristic
Example:
– If the length of an ear of corn was controlled by two genes Aa and Bb with A and B being dominant for long, the dihybrid cross would yield a range of phenotypes between pure dominant and pure recessive.
– The ear with the most dominant letters is the longest (AABB)
– What would the genotype be of the shortest ear?