Genetics
Revised By: Maria Johanna Baguio Testa
Monohybrid Cross
(Punnett Square)
Used to determine phenotype and genotype of offspring (F1 generation).
Phenotype = physical (color, shape, appearance)
Genotype = gene or allele (Aa, AA, aa)
Dominant = trait always displayed (AA, Aa)
Recessive = trait always masked; only expressed when homozygous (aa)
Allele Examples
AA - Homozygous Dominant
Aa - Heterozygous
aa - Homozygous recessive
Monohybrid Cross (Punnett Square)
Example: The trait for brown eyes is dominant to blue eyes (b). Show a cross between a parent (P1) with blue eyes, and a parent that is heterozygous for brown eyes.
P1: Bb x bb
F1: 50% Bb (brown eyes),
50% bb (blue eyes)
Co-Dominance
All traits are DOMINANT (no recessive; lowercase alleles)
In co-dominance, both traits are expressed.
Red Fish(R) x Blue Fish (B) = Red/Blue Fish
Co-Dominance and Blood Typing
Blood Type
Allele
A
AA (dominant) or Ai (heterozygous)
B
BB (dominant) or Bi (heterozygous)
AB
AB
O
ii
Always assume that in blood-typing, that the question is referring to the heterozygous allele (Ai or Bi), unless otherwise stated
Use the Punnett Square to determine blood type
Co-dominance and Blood Typing
What do you think? Is it possible for a mother with Type A blood and a father with Type B blood to produce a child with Type O blood?
Yes, it’s possible!
If both parents are heterozygous for the blood type
Incomplete dominance
All traits are DOMINANT (no recessive; lower cased alleles)
In incomplete dominance, traits blend together.
Ex. Red flowers (R) x White flowers (W) = Pink Flowers
Sex-Linked Traits
“X-Linked Traits”
Affects the sex chromosomes (pair #23)
Most Sex Linked Traits affect the X chromosome.
Males are more affected by sex-linked traits because they only have one X chromosome
Females are often carriers because they have two X chromosomes
Female Sex Chromosomes
XX
(carrier)
Male Sex Chromosomes
XY
(affected)
She has an extra X, so she’s safe
He only has one X, so he has the disease
Meiosis
Makes Gametes (sex cells)
Divides adult chromosome number in half (haploid); so that parents only pass 23 chromosomes to offspring.
Meiosis I: Homologous chromosomes
a) Crossing Over (swapping of chromosomes from mom & dad). Occurs in Prophase I
b) Law of Independent Assortment: states that alleles (genes) that you receive from your parents sort RANDOMLY; during Meiosis when your gametes are being made. The reason why you are genetically different, and look different from parents.
c) Non Disjunction (chromosomes do not separate properly), adding one chromosome, causing Down Syndrome, Klinefelter’s, Turner Syndrome
Meiosis II: Sister Chromatids
End result; four daughter cells with 23 chromosomes. Meiosis II and Mitosis are similar
Meiosis
Crossing Over
II
II
Four haploid daughter cells with 23 chromosomes
Chromosomes break apart, exchanging genetic material
Two diploid daughter cells with 46 chromosomes
Divides chromosomes number in half so there are only 23 chromosomes in gametes
23 mom + 23 dad = 46 you!
Law of Independent Assortment states that the chromosomes you get from mom and dad (23 Mom + 23 Dad) sort randomly as your gametes are being made during Meiosis
Events in meiosis
Crossing Over: Occurs in Prophase I, where the chromosomes break, and exchange genetic material
Why organisms look different than parents
Non-disjunction: failure of chromosomes to separate properly, causing organism to have extra chromosome (Trisomy 21)
Karyotypes and Genetic Disorder
Karyotypes are mapping of human chromosomes
A normal human karyotype has 46 chromosomes (diploid), and 23 pair (haploid).
Chromosomes 1-22 are called autosomes.
The sex chromosomes are the 23rd pair
Males are XY; females are XX
Normal Male Karyotype (XY)
Male (XXY) Klinefelter’s Syndrome
Males have an extra X Chromosomes
Smaller genital region
Develops breasts
47 chromosomes total
Klinefelter Male
Female Down Syndrome Karyotype
Trisomy 21
(Down Syndrome)
- Extra chromosome @ pair #21
- Down Syndrome (Trisomy 21)
Female Turner Syndrome (XO)
- Female with 45 chromosomes total.
- Missing and X chromosome.
- Infertile
- Does not go through puberty
- Non-functioning reproductive organs
Female (Turner Syndrome) XO
Cystic Fibrosis
- Thick mucus in the lungs and digestive tract
- Mainly affects Caucasian population
- 1 in 28 Americans carries the trait.
- Caused by defective protein in cell membrane.
- Treated with special diet, and physical therapy.
Tay-Sachs Disease
- Caused by recessive trait
- An important enzyme (protein) is missing that breaks down lipids (fat) in the central nervous system.
- Fat (lipids) accumulates in the CNS causing damage
Sickle Cell Anemia
- Primarily affects the African-American Community
- Red blood cells are crescent shaped
- Pain in the extremities
- Caused by Point (substitution) mutation
- Patients with SCA are immune to Malaria
PKU (Phenylketonuria)
- Recessive disorder
- Missing an enzyme that converts the amino acid Phenylalanine into Tyrosine.
- Phenylalanine cannot be broken down by the body and causes damage to the CNS
- Patients treated with diet low in phenylalanine
- Phenylalanine is found in many diet sodas.
Huntington’s Disease
- Degeneration of
- Nerve Cells (neurons)
- Cognitive impairment
- Inability to focus
- Muscle rigidity
- No treatment
- Death in 1-5 years
- Neuron Apoptosis
- (neurons die)
Pedigree Key
- Male
- Female
- Affected Female
- Affected Male
- Marriage (Union)
Carrier
A pedigree is a mapping of genetic inheritance. A family tree of disease.
Pedigrees and Genetic Inheritance
END