About 1 in 12 males has some form of color blindness.
Only 1 in 200 females are color blind.
Mendel and the Black Box
Gregor Mendel was the first person to comprehend some of the most basic principles of heredity.
Mendel reached these understandings in the mid 1800’s working in the Czech Republic with the garden pea, Pisum sativum.
Mendel performed his research prior to discoveries of chromosomes, DNA, genes, etc., focusing on paired genes on chromosomes: homologous & alleles separated during meiosis.
The Experimental Subjects
Mendel looked at seven characters in his pea plants—attributes such as seed color and texture.
Traits observed included which of those traits showed up in succeeding generations.
Cross Pollination
Mendel’s experiment involved cross-pollination between flowers grown from yellow seeds and flowers grown from green seeds.
The offspring resulted in yellow seeds, showcasing the dominance of the yellow trait over the green.
Phenotypes & Genotypes
Phenotype: any observable character.
Example: physical features, bodily characteristics, or behaviors of an organism.
In Mendel’s plants, purple flowers and white flowers are different phenotypes.
Genotype: the genetic makeup of an organism.
E.g., purple flower genotype (dominant) = W, white flower genotype (recessive) = w.
Each variable form of a gene is known as an allele.
Recessive allele (w) produces a white flower; dominant allele (W) produces a purple flower.
Dominant alleles determine a phenotype, regardless of the presence of recessive alleles.
Recessive alleles do not produce a noticeable effect on phenotype unless they are homozygous (two copies present).
With two alleles, three possible combinations exist: WW, Ww, and ww.
Homozygous: WW and ww have two copies of the same allele.
Heterozygous: Ww has two different alleles.
Three genotypes yield two phenotypes for seed color:
Examples: yellow seeds (Y) and green seeds (y).
Mendel’s Experiments
Mendel cross-pollinated peas that produced green seeds with those that produced yellow seeds.
Results: 78% of the offspring had yellow seeds.
This indicates that the yellow allele is dominant.
Important observation: Colors did not blend.
Inquiry: Did the green trait disappear?
Allowed the plants to self-pollinate.
Resulting generation produced a 3:1 ratio of yellow seeds to green seeds.
Inquiry: What is the explanation for this ratio?
Punnett Square
Punnett Square is a tool to show the genetic combinations from two parents during meiosis.
Example of alleles from a cross:
Parents: P (Yellow) and p (Green).
Offspring possibilities:
PP, Pp, pp.
The Punnett Square elucidates Mendel’s 3:1 ratio of yellow to green seeds.
Crosses with Two Characters
Mendel observed that genes for different characters passed on independently.
Example with smooth vs. wrinkled peas and yellow vs. green seeds.
P generation crossed:
Parents: SS YY (smooth yellow) and ss yy (wrinkled green).
F1 generation will produce:
Ss Yy (smooth yellow).
The segregation patterns show the independence of alleles on non-homologous chromosomes during meiosis.
Incomplete Dominance
Incomplete dominance occurs when neither allele for a given gene is completely dominant, resulting in a blended phenotype.
Example: pink snapdragons (cross between red and white).
Codominance
If differing alleles express equal effects in a single organism, this phenomenon is known as codominance.
This occurs in the gene coding for type A and B antigens in human blood.
Individuals with one A and one B allele exhibit type AB blood.
Neither allele is recessive; each exerts a separate phenotypic effect.
Polygenic Inheritance
Human beings can have no more than two alleles at a time for a given gene, with each allele residing on a separate, homologous chromosome.
However, many alleles can exist in a population for a given gene.
Example in plant height with three alleles: Tall, Medium, Very Short.
Most traits are governed by multiple genes, contributing to a polygenic inheritance.
Polygenic inheritance results in a character influenced by multiple genes, each exerting a small effect.
This leads to continuous variation in phenotypes, producing a bell-curve distribution in trait values.
Example: human skin color exhibits a range rather than distinct categories.
Continuous Variation and the Bell Curve
The effects of genes vary significantly based on the environment in which they are expressed.
Interaction of genotype and environment produces the observable phenotype.
X-Linked Inheritance
Certain human conditions, such as red-green color blindness and hemophilia, are known as X-linked conditions, stemming from alleles only located on the X chromosome.
Males are more likely than females to suffer from these conditions due to having only one X chromosome.
Male Gametes: The allele on the X chromosome does not have a homologous counterpart to compensate for its effects.
Female Gametes: Females have a pair of homologous X chromosomes, which allows the second allele to protect them from the effects of the deficient allele.
X-Linked Inheritance Examples
Examples of X-linked traits: hemophilia and color blindness.
Females can be carriers without showing symptoms if they have at least one functional allele.
Recessive alleles can still be passed on by carriers.
Carriers have one functional allele and one deficient allele.
Autosomal Genetic Disorders
Sickle-cell anemia is an example of an autosomal disorder caused by a genetic defect not involving the sex chromosomes.
Individuals must be homozygous for the sickle-cell allele to exhibit this condition — having two alleles for sickle-cell hemoglobin.
Risk of offspring with sickle-cell anemia is dependent on carrier status in parents:
If both parents are carriers, there is a 25% chance of Sickle Cell Anemia.
Sickle cell has unique characteristics: carriers (heterozygotes) are resistant to malaria, while homozygotes face decreased blood flow.
Autosomal Genetic Disorders
Not all genetic disorders are recessive; some are dominant.
Example: Huntington’s Disease, which affects brain function.
Pedigrees
To study human trait inheritance, scientists use pedigrees, diagrams representing family trees to observe disease patterns.
These pedigrees help determine if diseases are X-linked, autosomal, recessive, or dominant.
Pedigrees
By analyzing pedigrees, inherited diseases can be traced in family lines.
Patterns show affected and unaffected individuals across generations.
Genotypes can sometimes be inferred from pedigree analysis.
Conclusion
Understanding genetic inheritance through these principles provides insights into health, disease transmission, and the biological diversity observed within species.