Patterns of Inheritance PowerPoint detailed

PATTERNS OF INHERITANCE
Group 3: Zariah Beckford
Kerona Vassel
Bobbi-Gay Evans
Tanecia Hyatt
Dania McKenzie

DEFINITIONS
 Gene- a segment of DNA that is the basic unit of heredity and is passed down from
parents to their offspring.
 Allele- a variant of a gene, or a different version of a DNA sequence at a specific
location on a chromosome. Each person inherits two copies of a gene, known as
alleles, from each parent.
 Dominant- refers to the relationship between two versions of a gene, or alleles,
that are inherited from a parent
 Recessive-a genetic trait that is only expressed when an organism has two copies
of a recessive allele for that trait
 Codominant- a type of inheritance where two different versions of a gene, called
alleles, are expressed equally and produce different traits in an organism.

DEFINITIONS CONT’D
 Codominant con’td- In codominance, neither allele is dominant or recessive, and
both traits appear. For example, a plant or animal with more than one pigment
color exhibits codominance. Blood types: In the ABO blood group system, people
with type AB blood have one allele for A and one for B.
 Multiple alleles- three or more different types of alleles for a given trait in a
population.This is different from the idea that each gene has only two alleles, one
dominant and one recessive, as proposed by Mendel.
 Dominant epistasis- a type of gene interaction that occurs when a dominant allele
of one gene masks the expression of all alleles of another gene. Example:
Albinism:The albinism gene is dominant and hides the expression of pigment in
the eyes, skin, and hair.

COMPARISON BETWEEN MONOHYBRID & DIHYBRID
CROSSES
A Monohybrid Cross
 a genetic cross between two
individuals that examines the
inheritance of a single trait,
involving two alleles (dominant
and recessive). It helps predict
the genetic variation in offspring
based on Mendel's laws of
inheritance. For example,
crossing a plant with a dominant
flower color allele with one that
has a recessive allele.
A Dihybrid Cross
 a genetic cross that examines the
inheritance of two traits, each with
two alleles (dominant and
recessive). It tracks how these
traits segregate and assort
independently, following Mendel's
law of independent assortment.
For example, crossing two pea
plants, one with dominant traits for
yellow, round seeds and the other
with recessive traits for green,
wrinkled seeds.

WHAT IS MENDEL’S RATIO?
Gregor Mendel discovered patterns in how traits are inherited.
- For two traits, Mendel predicted a 9:3:3:1 ratio:
- 9 with both dominant traits.
- 3 with one dominant and one recessive trait.
- 3 with the other dominant and recessive trait.
- 1 with both recessive traits.
- Example: Crossing plants with purple vs. green stems and cut vs.
potato leaves.

MENDELIAN DIHYBRID CROSS OVERVIEW
Gregor Mendel’s dihybrid cross experiments studied the
inheritance of two traits simultaneously, such as seed shape
(round or wrinkled) and seed color (yellow or green). Each trait
is governed by a pair of alleles:
• Seed Shape: Round (R) is dominant to Wrinkled (r).
• Seed Color: Yellow (Y) is dominant to Green (y).
A dihybrid organism is heterozygous for both traits (RrYy).
When two dihybrid organisms are crossed, the offspring follow a
9:3:3:1 phenotypic ratio, assuming independent assortment of
alleles.

CONT’D
 Steps in Analysis
1.Set Up the Cross:
1. Parental genotype: RrYy × RrYy
2. Gametes: Four types from each parent—RY, Ry, rY, and ry.
2.Construct a Punnett Square:
1. Use a 4x4 Punnett square to represent all possible offspring genotypes.
3.Phenotypes and Ratios:
1. Dominant Traits: Round (R) and Yellow (Y)
2. Recessive Traits: Wrinkled (r) and Green (y)
3. Offspring Phenotypes and Ratios:
1. 9 Round Yellow (RRYY)
2. 3 Round Green (RRyy)
3. 3 Wrinkled Yellow (rrYY)
4. 1 Wrinkled Green (rryy)

CONT’D
Experimental Analysis of Seed Samples
1.Collect Seed Samples:
•Observe and count seeds for the different phenotypes (RoundYellow, Round Green,
WrinkledYellow,Wrinkled Green).
2.Compare with Expected Ratios:
•The expected 9:3:3:1 ratio should be observed in a sample of seeds.
•For 160 seeds:
•RoundYellow: 90 seeds (9/16 of total)
•Round Green: 30 seeds (3/16 of total)
•WrinkledYellow: 30 seeds (3/16 of total)
•Wrinkled Green: 10 seeds (1/16 of total)
This analysis confirms Mendel's principles of independent assortment and inheritance.

THE LAW OF SEGREGATION
The Law of Segregation states that an individual inherits one allele from each parent, resulting
in two alleles for a particular trait. During meiosis, which is the type of cell division that
produces gametes, these two alleles separate, with each gamete receiving only one allele.
When gametes from two parents combine during fertilization, the offspring inherits one allele
from each parent, restoring the allele pair.
For example, let's consider a gene for flower color with two alleles:
•R (dominant, red flowers)
•r (recessive, white flowers)
A plant with the genotype Rr (heterozygous) will produce gametes in equal numbers:
•Half will carry the R allele.
•Half will carry the r allele.
If this plant self-pollinates or mates with another plant, the potential offspring could inherit
combinations of alleles such as:
•RR (red flowers)
•Rr (red flowers)
•rr (white flowers), depending on the allele combination.

THE LAW OF INDEPENDENT ASSORTMENT
The Law of Independent Assortment states that genes for different
traits are inherited independently of each other, as long as they are
located on different chromosomes or far apart on the same
chromosome.This means the inheritance of one trait does not
influence the inheritance of another.
For example, consider a plant with two traits: seed color (Y for yellow,
y for green) and seed shape (R for round, r for wrinkled). A plant with
the genotype YyRr produces gametes with all possible combinations:
YR,Yr, yR, and yr.These combinations assort independently, resulting
in genetically diverse offspring.This principle is often illustrated using
a dihybrid cross Punnett square, which shows how different traits can
be inherited together.

THE CONCEPT OF PROBABILITY IN
GENETICS
 Data Table for Monohybrid Cross
 Ratios:
 Genotypic Ratio = 1 TT : 2 Tt : 1 tt
 Phenotypic Ratio = 3 Tall : 1 Short
Parental genotype Tt × Tt
Possible gametes T, t
Punnett
Square
T t
T TT Tt
t Tt tt

USE OF 0.05 CONFIDENCE LIMITS AND
NULL HYPOTHESIS
Terms Definition
Null Hypothesis (H )
₀ States there is no significant difference
between observed and expected results.
Alternative Hypothesis (H )
₁ States there is a significant difference
between observed and expected results.
0.05 Confidence Limit Indicates a 5% probability of results
occurring due to chance.

USE OF 0.05 CONFIDENCE LIMITS AND
NULL HYPOTHESIS
 The **0.05 confidence limit** (or significance level) is commonly used in hypothesis
testing to determine whether the results of an experiment are statistically significant. If the
p-value is less than 0.05, it indicates strong evidence against the null hypothesis,
suggesting the observed effect is unlikely to have occurred by chance.
 The **null hypothesis** is a statement that there is no effect or no difference in the
population being studied. It is typically tested against an alternative hypothesis. If the p-
value is greater than 0.05, the null hypothesis is not rejected, indicating insufficient
evidence to support the alternative hypothesis.
 In summary, a **0.05 confidence limit** helps assess the likelihood that an observed
result is due to chance, with results below 0.05 providing evidence to reject the null
hypothesis.

UNDERSTANDING THE CHI-SQUARE TEST
What is the Chi-Square Test?
-Helps determine if differences are due to chance or something else
 - Compares observed results with expected results.
 -A statistical tool used in genetics.

STEPS TO PERFORM THE CHI-SQUARE TEST
1. Set up your data: List observed (O) and expected (E) values.
2. Use the formula:
² = ((O - E)² / E)
χ Σ
3. Calculate: Perform calculations for each category and add them up.
4. Interpret: Compare ² to the critical value to check if the difference
χ
is significant.

EXAMPLE DATA FROM A GENETIC CROSS

RESULTS
Step-by-Step Calculation
 1. Purple stems, cut leaves:
 - O = 86, E = 81
 - (O - E) = 86 - 81 = 5
 - (O - E)² = 5² = 25
 - (O - E)² / E = 25 / 81 ≈ 0.31
 2. Repeat for all categories and calculate ²:
χ
- ² = ((O - E)² / E) = 0.31 + 0.04 + 0.33 +
χ Σ
0.11 = 0.79.
 - Add all results from the calculations:
 ² = 0.31 + 0.04 + 0.33+ 0.11 = 0.79
χ
Understanding the Results
 1. Degrees of Freedom:
 - Number of categories - 1 = 4 - 1 = 3.
 2. Critical Value:
 - From Chi-Square table: 7.81 (at 0.05
level).
 3. Compare Values:
 - ² = 0.79, Critical Value = 7.81.
χ
 - 0.79 < 7.81 Difference is NOT
→
significant.
 4. Conclusion:
 - Results match Mendel ratios;
differences due to chance.

KEY POINTS!
 Mendel’s Ratio helps predict offspring traits.
 Chi-Square Test checks if results match predictions.
 Gene: A segment of DNA that codes for a protein or polypeptide, found at a
specific locus on a chromosome. It can have different forms called alleles.
 Diploid Cells: Contain two copies of each gene. If both copies are the same, the
organism is homozygous; if different, it's heterozygous.
 Haploid Cells: (e.g., gametes) contain one copy of each gene.
 Dominant and Recessive Alleles: A dominant allele affects the phenotype even if
only one copy is present. A recessive allele only affects the phenotype when no
dominant allele is present. Codominant alleles have equal effects when both are
present.

KEY POINTS! CONT’D
 Multiple Alleles: Some genes have more than two alleles, like the ABO blood groups.
 Genetic Diagrams: These diagrams show possible genotypes in offspring and the
probabilities of their occurrence.
 Sex-Linked Genes: Genes on the X chromosome, which may affect males more than
females because males have only one X chromosome. Examples include haemophilia
and color blindness.
 Dihybrid Cross: A cross involving two genes. If one parent is homozygous for both
genes and the other is heterozygous, the offspring ratio is 1:1:1:1. If both parents are
heterozygous, the ratio is 9:3:3:1.
 Epistasis: When the expression of one gene is influenced by another gene.
 Chi-Square Test: A statistical test used to determine if the difference between
observed and expected genetic results is significant. A p-value greater than 0.05
means there's no significant difference, so the null hypothesis is accepted

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