Genetic Testing

🧬 Genetics Test Study Guide

🔹 Monohybrid & Dihybrid Crosses

• Monohybrid: Cross involving one trait (e.g., Tt x Tt). Use Punnett square. Ratios often 3:1 (phenotype).
  
  

• Dihybrid: Involves two traits (e.g., RrYy x RrYy). Use 16-box Punnett square. Typical phenotype ratio: 9:3:3:1.
  
  

🔹 Pedigrees

• Circles = females, squares = males, shaded = affected.
  
  

• Dominant: Trait appears in every generation.
  
  

• Recessive: Skips generations.
  
  

• Sex-linked: Often more males affected.
  
  

• Autosomal Recessive: Must have two copies to express; can be passed by carriers (heterozygous mother can pass it to children).
  
  

🔹 Karyotypes

• Visual of chromosomes; shows number and structure.
  
  

• Can detect disorders like Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter (XXY).
  
  

• Shows sex chromosomes: XX = female, XY = male.
  
  

🔹 Sex-Linked Traits

• X-linked: Traits on X chromosome; males (XY) more affected.
  
  

• Y-linked: Only passed father → son (e.g., Y chromosome traits).
  
  

🔹 Non-Disjunction

• Failure of chromosomes to separate in meiosis.
  
  

• Results in gametes with extra or missing chromosomes → disorders like trisomy.
  
  

🔹 Meiosis vs. Mitosis

• Meiosis: 4 unique gametes (haploid), includes crossing over.
  
  

• Mitosis: 2 identical cells (diploid), no crossing over.
  
  

🔹 Polygenic Traits

• Controlled by many genes → wide range of phenotypes (e.g., height, skin color).
  
  

• Graph: Bell curve distribution.
  
  

🔹 Multiple Alleles

• More than two allele options exist (e.g., blood types: A, B, O).
  
  

🔹 Blood Type Inheritance

• Type A: IAIA or IAi
  
  

• Type B: IBIB or IBi
  
  

• Type AB: IAIB (codominant)
  
  

• Type O: ii (recessive)
  
  

🔹 Genetic Ratios and Gametes

• Dihybrid cross = 4 possible gametes per parent (FOIL method).
  
  

• Ratios from Punnett square show probabilities.
  
  

🔹 Heterozygous Advantage

• Carrier of recessive allele has survival benefit.
  
  

• Example: Sickle cell trait (AS) protects from malaria.
  
  

🔹 Tay-Sachs Disease

• Autosomal recessive.
  
  

• Carriers (heterozygous) unaffected but can pass it on.
  
  

• Allele persists due to possible heterozygous advantage.
  
  

How to do monohybrid crosses
Use a 2x2 Punnett square to cross two individuals with one trait (e.g., Tt × Tt). Genotype ratio: 1:2:1; Phenotype ratio: 3:1 (dominant:recessive).

What crossing over does
During meiosis, homologous chromosomes exchange genetic material, increasing genetic diversity in gametes.

What kind of genetic disorders can be determined by a karyotype
Chromosomal abnormalities like Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY), deletions, or duplications.

Karyotype sex chromosomes
Shows the 23rd pair: XX = female, XY = male. Abnormal combinations (e.g., XO, XXY) indicate disorders.

What a sex-linked gene is
A gene located on a sex chromosome (usually X); males (XY) are more affected due to having only one X.

How to determine if a trait is dominant or recessive using a pedigree
Dominant: Trait shows in every generation.
Recessive: Can skip generations.
Sex-linked: Often more males affected.

How non-disjunction can affect the number of chromosomes in gametes
It causes gametes to have too many or too few chromosomes, leading to disorders like trisomy 21 or monosomy.

How a sex-linked trait would be inherited on the Y chromosome
Only males inherit Y-linked traits, passed directly from father to son.

If a mother has an autosomal recessive disorder, can it be passed to her offspring?
Yes. All offspring will inherit one copy of the gene; if the father also contributes a recessive allele, children can be affected.

How to do a dihybrid cross, how to determine ratios
Cross two individuals with two traits (e.g., RrYy × RrYy). Use a 16-square Punnett square. Typical phenotype ratio: 9:3:3:1.

How to determine the number of different gametes that can be produced in a polygenic trait
Use 2^n, where n is the number of heterozygous gene pairs. More genes = more variation in gametes.

The products of meiosis
4 genetically different haploid gametes (sperm or eggs), each with half the original number of chromosomes.

Differences between mitosis and meiosis

• Mitosis: 2 identical diploid cells; growth/repair.
  
  

• Meiosis: 4 unique haploid gametes; sexual reproduction.
  
  

• Meiosis includes crossing over and two divisions.
  
  

What a graph of a polygenic trait will look like
A bell-shaped curve (normal distribution) showing continuous variation.

What type of inheritance blood type is, how it works

• Codominance (A and B alleles both show in AB).
  
  

• Multiple alleles: A, B, and O.
  
  

• Inheritance: IA, IB are dominant over i.
  
  

How multiple alleles function
More than two forms of a gene exist in the population (e.g., A, B, O for blood type), though individuals still have only two alleles.

How to read a pedigree

• Circles = females, Squares = males.
  
  

• Shaded = affected.
  
  

• Use generations and patterns to identify inheritance (dominant/recessive, autosomal/sex-linked).
  
  

What a karyotype is, what they can show
A visual chart of all chromosomes. Can reveal chromosome count, sex, and major structural changes (e.g., trisomies, deletions).

What heterozygous advantage is, examples
A carrier (heterozygote) has a survival benefit.
Example: Sickle cell trait protects against malaria.

How Tay-Sachs disease is inherited, why its allele remains in population

• Autosomal recessive: Both parents must carry the allele.
  
  

• The allele persists possibly due to heterozygous advantage (e.g., carriers may have resistance to tuberculosis).