Mendel and the Gene Idea

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Mendel and the Gene Idea

• Chapter 14

2
Terminology

• Alleles dominant and recessive
• Homozygous
• Heterozygous
• Phenotype
• Genotype
• P1 F1 F2
• Test cross

3
Mendels Laws

• Law of Segregation
• Law of Independent Assortment

4
Rules of Probability

• Probabilities of all event must add to 1
• Example
• Toss a coin ½ heads 1/2 tails 1
• Random events are independent of one another

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Rule of multiplication

• the probability that independent events will
  occur simultaneously is the product of their
  individual probabilities
• Parents Pp x Pp
• What is the probability offspring pp?
• ½ x ½ ¼

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Dihybrid Crosses

• YyRr x YyRr probability of YYRR ?
• ½ x ½ ¼ probability of YR from one parent
• ½ x ½ ¼ YR from other parent
• ¼ x ¼ 1/16 from both parents

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Rule of Addition

• Event can occur in two or more different ways
• Sum of separate probabilities
• Pp x Pp ? Chance of offspring is Pp
• ½ x ½ ¼ (P from egg, p from sperm)
• ½ x ½ ¼ (p from egg, P from sperm)
• ¼ ¼ ½

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Practice

• What fraction of offspring would show 2 recessive
  phenotypes? PpYyRr x Ppyyrr
• ppyyRr ¼ x ½ x ½ 1/16
• ppYyrr ¼ x ½ x ½ 1/16
• Ppyyrr ½ x ½ x ½ 2/16
• PPyyrr ¼ x ½ x ½ 1/16
• ppyyrr ¼ x ½ x ½ 1/16 equals 6/16 3/8

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Variations from Mendelian Genetics

• Ch. 14

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Variations from Mendelism

• 1. INCOMPLETE DOMINANCE
• Dominant phenotype is not fully expressed
• Red (RR) x white (rr) P1
• F1 all pink (Rr)
• Rr x Rr
• 1 red, 2 pink, 1 white

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Incomplete dominance
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2. Codominance

• Full expression of both alleles
• Blood groups each allele expressed
• A, B, AB, O
• 3. MULTIPLE ALLELES more than just two
  alternative forms of gene
• Individual inherits only two
• Blood groups

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Inheritance of Blood type

• Genotype antigens antibodies type
• IAIAor IAi A B A
• IBIBor Ibi B A B
• IAIB A and B none AB
• ii none A B O
• Universal donor?
• Universal recipient?

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Codominance
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4. Pleitropy

• Single gene has multiple phenotypic effects
• Sickle cell anemia
• Gene for fur pigmentation influences connection
  between cats eyes and brain
• 5. EPISTASIS stand upon
• One gene pair modifies expression of another gene
  pair

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Epistasis
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Examples

• Coat color in rodents
• Must have dominant gene for melanism
• Must have dominant gene for coat color
• BbCc, BbCC, BBCc, BBCC black
• bbCc, bbCC brown
• Bbcc, bbcc white

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Pleiotropy
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6. Polygenic Inheritance

• Many genes govern one trait
• Continuous distribution rather than discrete
  differences
• Additive effect of two or more genes determines
  single phenotypic character
• Skin color
• Environmental Influence on genetics ?????

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Mendelian Inheritance

• In Humans

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Pedigree Analysis
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Autosomal Recessively Inherited Disorders list
only (each will be discussed in detail)

• Cystic Firbrosis Tay-Sachs
• Sickle-cell disease PKU
• Albinism
• Blue offspring
• Ellis-Van Crevels Syndrome extra fingers, toes,
  short limbs (Amish)

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Recessively Inherited Disorders

• 1. Cystic Fibrosis
• Most common lethal genetic disease in US
• 1/2,500 Caucasians
• 4 of Caucasians carriers
• Recessive for allele that controls chloride
  traffic across membrane
• Build up of mucous

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2. Tay-Sachs

• 1/3,600 births
• Ashkenazi Jews (Europe) and Mediterranean Jews
• Brain cells unable to metabolize a lipid
• Builds up in brain
• Infant deteriorates until death at age 3 or 4
• Can be tested to determine if carrier

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3. Sickle-cell Anemia

• 1/400 African-Americans
• Caused by single amino acid substitution in
  hemoglobin
• Pleiotropy
• RBC changes shape many symptoms
• Heterozygotes enhanced resistance to malaria

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Consanguinity

• Genetic relationship from shared ancestry
• Results in harmful recessive homozygotes
• Most cultures forbid marriage between relatives

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Autosomal Dominantly Inherited Disorders

• 1. Achrondroplasia
• 2. Achoo syndrome chronic sneezing
• 4. Polydactly
• 5. Progeria premature aging
• LateActing lethal dominants
• 1. Huntingtons Disease

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Huntingtons Disease

• Degenerative disease of nervous systems
• Symptoms appear between 35-40 yrs.
• Irreversible and fatal
• Child has 50 chance of inheriting
• Now possible to test for presence of allele

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Penetrance and expressivity

• Not all traits express 100 even though allele is
  present
• P polydactly in humans
• Two normal adults offspring with extra digits
• Genotype of one parent?
• Pp shows reduced penetrance of P allele

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Penetrance

• Frequency of expression of an allele when it is
  present in the genotype
• 90 penetrance 9/10 of individuals with allele
  will express it
• P allele has variable expressivity
• Expressivity variation in allelic expression
• Can occur on one or more appendages
• May be full size or stub

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Polydactly in blues guitaris Hound Dog Taylor
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Testing for genetic disorders

• Carrier Recognition
• Fetal Testing
• Amniocentesis
• Chorionic villus sampling
• Ultrasound
• Fetoscopy
• Newborn screening for PKU