Complex Patterns of Inheritance, Blood Groups, and Chromosomal Abnormalities

Incomplete Dominance and Intermediate Phenotypes

Definition of Incomplete Dominance: This occurs when one allele does not have complete dominance over another, resulting in an intermediate phenotype where traits are partially expressed.
The Heterozygous State: In incomplete dominance, the intermediate phenotype is always seen in the heterozygote. While parents may be homozygous for their respective traits, the offspring inherit a blend of these parental phenotypes.
Example: Carnations and Snapdragons: * Carnations can exhibit red or white phenotypes. * When crossed, neither trait is completely dominant. * The alleles are often represented by capital letters (e.g., $R$ and $W$ ). * The resulting intermediate phenotype in the F1 generation is pink.
Genetic Representation of Snapdragons: * Homozygous Red: $RR$ * Homozygous White: $WW$ * Heterozygous Pink (Intermediate): $RW$
Inheritance Ratios in Incomplete Dominance: * A cross between homozygous red ( $RR$ ) and homozygous white ( $WW$ ) parents will result in $100\%$ heterozygous pink ( $RW$ ) offspring. * If two heterozygous pink carnations ( $RW \times RW$ ) are crossed, the resulting phenotypic and genotypic ratio is $1:2:1$ . * Expected outcome: $1$ Red ( $RR$ ), $2$ Pink ( $RW$ ), and $1$ White ( $WW$ ). * This differs from the standard Mendelian monohybrid ratio of $3:1$ seen in complete dominance.

Codominance in Genetics

Definition of Codominance: Unlike incomplete dominance, codominance involves both alleles being expressed equally and simultaneously in the phenotype. There is no "blending" of traits; instead, both parental traits are visible.
Example: Flowers: In a codominant scenario for carnations, a cross between red and white parents would result in offspring with patches of red and patches of white, rather than a solid pink color.
Example: Cattle Coat Color: * Red coat: $RR$ * White coat: $WW$ * Roan coat ( $RW$ ): This is the intermediate codominant phenotype where the calf has distinct patches or hairs of both red and white.
Inheritance Patterns: * Crossing homozygous red cattle with homozygous white cattle results in all heterozygous roan ( $RW$ ) offspring in the F1 generation. * Crossing two roan individuals ( $RW \times RW$ ) results in the same $1:2:1$ ratio: $1$ Red ( $RR$ ), $2$ Roan ( $RW$ ), and $1$ White ( $WW$ ).
Human Example: AB Blood Type: The AB blood type is a prime example of codominance where both the A and B antigens are expressed equally on the surface of red blood cells.

Multiple Alleles and the ABO Blood Group System

Mendel’s Limitations: Gregor Mendel originally suggested that there were only two alleles for each gene, but modern genetics shows that many genes have multiple alleles within a population.
The ABO System: * Blood groups are determined by a single gene but controlled by three different alleles: $I^A$ , $I^B$ , and $I^O$ (sometimes written as simple $A, B, O$ ). * The groups are categorized by glycoproteins (antigens) found on the surface of red blood cells. * Allele $I^A$ : Produces antigen A. It is codominant with allele $I^B$ . * Allele $I^B$ : Produces antigen B. It is codominant with allele $I^A$ . * Allele $I^O$ : Does not produce any antigen. It is recessive to both $I^A$ and $I^B$ .
Genotypes and Phenotypes: There are six possible genotypes leading to four distinct phenotypes: * Type A: $I^A I^A$ or $I^A I^O$ * Type B: $I^B I^B$ or $I^B I^O$ * Type AB: $I^A I^B$ * Type O: $I^O I^O$

Blood Transfusions and Compatibility

Mechanism of Rejection: Blood transfusions must be matched based on the antigens on the donor's cells and the antibodies in the recipient's plasma. If they do not match, the recipient's immune system will reject the blood.
Universal Donor: Type O is considered the universal donor because it lacks A and B antigens, meaning it will not trigger an antibody response in recipients of other blood types.
Universal Recipient: Type AB is the universal recipient because these individuals do not have antibodies against A or B antigens (as they possess both antigens themselves).
Specific Rules: * Type A cannot receive B or AB blood. * Type B cannot receive A or AB blood. * Type A can receive A or O.

Sex Determination and Chromosomes

Human Karyotype: Humans typically have $23$ pairs of chromosomes ( $46$ total). The first $22$ pairs are homologous autosomes, while the $23$ rd pair consists of sex chromosomes.
Biological Sex: * Females: $XX$ * Males: $XY$
Fertilization: Biological sex is determined by the sperm. If a Y-containing sperm fertilizes the egg ( $X$ ), the offspring is male ( $XY$ ). If an X-containing sperm fertilizes the egg, the offspring is female ( $XX$ ).
Platypus Trivia: The speaker notes that platypuses have a complex sex chromosome system consisting of $10$ sex chromosomes ( $5X$ and $5Y$ for males, or $10X$ for females).

Chromosomal Abnormalities and Non-disjunction

Mechanism of Non-disjunction: This occurs when chromosomes or sister chromatids fail to separate properly during Meiosis I or Meiosis II. This results in gametes with an abnormal number of chromosomes (aneuploidy).
Examples of Syndromes: * Down Syndrome: Trisomy $21$ . The individual has $47$ chromosomes with an extra copy of chromosome $21$ . This often results from non-disjunction in the mother's egg cell. * Klinefelter Syndrome: An individual has an $XXY$ genotype ( $47$ chromosomes). They generally present with male characteristics but are often sterile. * Turner Syndrome: An individual has a single X chromosome ( $XO$ genotype, $45$ chromosomes). They present with female characteristics but are generally sterile.

Sex-Linked Inheritance and Diseases

Structural Difference between X and Y: The Y chromosome is significantly smaller than the X chromosome. Therefore, many genes located on the X chromosome do not have a corresponding allele on the Y chromosome.
Expression in Males: Because males only have one X chromosome, any allele present on that X (whether dominant or recessive) will be expressed in the phenotype. This is why sex-linked diseases are more common in males.
Red-Green Color Blindness: * Normal vision allele: $X^C$ * Color blind allele: $X^c$ * Possible Genotypes/Phenotypes: * $X^C X^C$ : Normal female * $X^C X^c$ : Carrier female (normal vision) * $X^c X^c$ : Color blind female (rare) * $X^C Y$ : Normal male * $X^c Y$ : Color blind male
Hemophilia: A sex-linked blood clotting disorder that follows the same inheritance pattern as color blindness.
Crossing Ratios: In a cross between a carrier female ( $X^C X^c$ ) and a normal male ( $X^C Y$ ), there is a $50\%$ chance that a son will be color blind and a $50\%$ chance that a daughter will be a carrier.

Questions & Discussion

Question (Audience): If O is recessive, why is it the most common blood type in the population?
Response: This is linked to human migration and gene flow. While O is recessive, it became predominant in certain populations (originating in Africa) as they migrated and interbred. In other regions, like the Middle East, it may not be the most common type. Genetic dominance does not equal population frequency.
Question (Audience): Is non-disjunction (like Klinefelter or Turner syndrome) the same as being intersex?
Response: No. These are specific genetic syndromes. While there are similarities, intersex conditions often involve a biological expression of both male and female traits (similar to codominance), such as a biologically male individual possessing parts of a womb. Individuals with Klinefelter or Turner transition are typically sterile and present as primarily male or female.
Dialogue Note on Personal Experience: A student shared an anecdote about an intersex individual who discovered they had parts of a womb during medical testing as a child, highlighting the complexity of biological sex beyond standard karyotypes.