gene_2.pdf

Monohybrid Crosses
Genetic (Osamah Alrouwab) 1

Monohybrid Crosses

Dihybrid Crosses

2- Non-Mendelian Inheritance
Some Genetics Traits Grows beyond Mendel’s !!!
While the majority of pairings resulted in offspring with genotypic and phenotypic ratios
consistent with Mendel's rules, certain aberrations were observed. These natural
anomalies illustrate non-Mendelian inheritance. Natural variations fall into two categories:
1. Mendel's laws irrelevant:
Both genotypic and phenotypic ratios deviate from those predicted by Mendel's rules
2. Genotypic ratio is that expected from mendel’s laws but the phenotypic ratio is not

Mendel's experiments were not seen as outliers due to :
1. All the traits he examined showed dominant or recessive.
2 .Each trait was determined by single gene (two alleles).
3. These alleles assorted independently.

Mendel's experiments were not seen as outliers due to :
1. All the traits he examined showed dominant or recessive.
2 .Each trait was determined by single gene (two alleles).
3. These alleles assorted independently.
❑ In 1902, enthused British biologist William Bateson published an English translation of Mendel’s
German text and appended a brief account of what he called “Mendelism—the Principles of
Dominance, Segregation, and Independent Assortment.” Later, in 1909, he published Mendel’s
Principles of Heredity.
❑ Laterally, the scientists observed bot all traits are controlled by single gene nor are all genes
represented by two alleles.

traits that considered as deviation of Mendelian inheritance can be
summarized into:
1- Incomplete Dominance
2-Codominance.
3- Multiple Alleles.
4- Lethal alleles .
5- Polygenic Inheritance.
6- Pleiotropic ("Multiple Phenotypes").
7- Sex-Linked Traits.

1- INCOMPLETE DOMINANCE
❑Also knows as partial Dominance
❖ occasionally, when the progeny is heterozygote (e.g. Ww)
and • generated from homozygote parents (one is dominant
and the other is recessive, e.g. WW x ww), the heterozygote
phenotype appears to be an intermediate trait between the
two parents because one allele is not completely dominant
over the other and partial expression of both traits occurs,
resulting in the appearance of a new third phenotype (as
blend of dominant and recessive).
❖ The dominant allele cannot mask the other allele's expression. Thus, heterozygous
parents' phenotypes are intermediate to those of homozygous parents.
flower color of snapdragons

flower color of snapdragons

X

2- Codominance
❑ The state of expression of both alleles in a heterozygous organism.
BOTH alleles are involved in the phenotype.
❑ The two alleles do not coexist, but are both present in children with neither
dominant nor recessive phenotypes.

2- Codominance
RR= Red Flower WW= White Flower RW= Red and White Flower
X

2- Codominance

3- Multiple Alleles
❖ Most of the genetic systems that we have examined so far consist of two alleles.
❖ ln Mendel's peas, for instance, one allele encoded round seeds and another encoded
wrinkled seeds.
❖ For some loci, more than two alleles are present within a group of organisms--- the locus
has multiple alleles (multiple alleles may also be referred to as an allelic series).

3- Multiple Alleles
1- DUCK-FEATHER PATTERNS : An example of multiple
al leles is at a locus that determines the feather pattern of mal
lard ducks. One allele M produces the wild-type mallard
pattern. A second allele MR produces a different pattern called
restricted, and a third allele md produces a pattern termed
dusky. In this allelic series restricted is dominant over mallard
and dusky, and mallard is dominant over dusky MR > M > md
The six genotypes possible with these three alleles and their
resulting phenotypes are:

3- Multiple Alleles
ABO blood group system

4- Lethal alleles
❑ A lethal allele causes death at an early stage of development often before birth, and
so some genotypes are not appear among the progeny

5- Polygenic Inheritance
❑ Greek origin poly, meaning “many and genic, meaning “of genes”.
❑ A single trait controlled by many genes is called polygenic trait and inheritance of
such trait is called polygenic inheritance.
❑ genes of a polygenic trait follow Mendel's laws but together do not produce Mendelian
ratios.
1- Body Type
2- Height
3- Skin Color
4- Hair color
5- Eye color
6- Intelligence
Some examples of polygenic traits
❑ Polygenic traits are also called continuously varying traits.

❑ Human skin color ranges in variety from darkest brown
to lightest fair color (almost white).
1- Human skin color
❑ Melanin is the pigment that determine skin color in
humans.
❑ Melanocytes Melanin Melanogenesis
❑ In humans the melanin production is controlled by
interaction of three separate genes A, B and C located
on different chromosomes.

❑ A person with the alleles AABBCC is very dark skinned
1- Human skin color
❑ A person with the alleles aabbcc is very light skinned

1- Human skin color
6 Dominant ➔ black
5 Dominant ➔ very dark
4 Dominant ➔ dark
3 Dominant ➔intermediate
2 Dominant ➔fair
1 Dominant ➔very fair
0 Dominant ➔ albino

6- Pleiotropic ("Multiple Phenotypes")
Pleiotropy ➔ results when a single gene affects more than one trait.
Example:
Sickle cell disease
Causes sickle-shaped blood cells, contributes to organ damage,
& imparts resistance to malaria.

7- Sex-Linked Traits
Alleles are carried on the X chromosome
Example: Red-green colorblindness
B = normal b = colorblind
Males Females
XY XX
XY XX
XX

Chromosomal abnormalities
Trisomy 21, Down Syndrome
(a) Extra or missing chromosomes cause
disorders.
Monosomy: only 1 of 2 chromosomes present
Trisomy: 3 chromosomes present (instead of 2)

Turner’s Syndrome, one X chromosome

Cri du Chat, small deletion of chromosome #5
(a) Missing parts of a chromosome
Deletion, Inversion, Translocation
• Many survive to
adulthood with
marked disability

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