Mendelian and Non Mendelian Genetics: Monohybrid and Dihybrid Crosses, Blood Types, Sex-Linked Traits, and Pedigrees

Join Amoeba Sisters for a clear, hands-on review of genetics that starts with Mendelian one-trait crosses and dihybrid crosses, then explores non Mendelian patterns such as incomplete dominance and codominance, plus multiple alleles and sex linked traits. The video uses engaging examples like hair phenotype in guinea pigs, ABO blood types, and hemophilia, to illustrate how Punnett squares translate genotypes into phenotypes and how pedigrees help track traits across generations. It also emphasizes problem-solving strategies, the probabilistic nature of genetics, and the importance of practice and careful note-taking. Expect concrete steps, visual aids, and practical tips for solving classic genetics problems.

Introduction

The Amoeba Sisters present a structured genetics review that moves from classical Mendelian patterns to non Mendelian inheritance and pedigrees. The goal is to build familiarity with problem solving in biology class, including Punnett squares, genotype and phenotype ratios, and interpreting pedigrees. A key message is that Punnett squares predict probabilities, not guaranteed outcomes, and real genetic results follow probabilistic principles that average out over many offspring.

Mendelian Genetics: One-Trait Cross (Monohybrid)

The video introduces a simple monohybrid cross using a guinea pig example where a dominant allele is associated with hair and a recessive allele with hairlessness. Each guinea pig has two alleles, one from each parent. The instructor guides the viewer through a Punnett square for a cross between a hairless (hh) and a heterozygous (Hh) individual. The offspring show a 1 to 1 genotype ratio (2 Hh to 2 hh) and a 1 to 1 phenotype ratio (2 with hair to 2 hairless). The takeaway is that genotype and phenotype ratios do not always match, but in this Mendelian monohybrid example they do. This section reinforces the concept that genetic problems are about probabilities across offspring rather than certainties in any single litter.

"Remember it only takes one dominant allele and it would have hair." - Sam

Mendelian Genetics: Two-Trait Cross (Dihybrid)

The discussion then moves to a dihybrid cross, using cats with two fictional traits: hair presence (H) and fondness for sinks (S). The parental genotype is written as Big H little H, Big S little S for both parents. To perform the cross, the foil method is used to generate gamete combinations on the sides of a 16-cell Punnett square. The resulting phenotypic ratio for a dihybrid cross of heterozygotes is the classic 9:3:3:1. The lesson emphasizes how examining two traits together reveals linked probabilities across four gamete combinations. The example also notes that different trait combinations may produce different patterns depending on the specific alleles involved.

"You will find that a dihybrid cross between heterozygotes for both traits yields a 9:3:3:1 phenotype ratio." - Sam

Non Mendelian Inheritance

Moving beyond classic Mendelian rules, the video covers incomplete dominance and codominance. In incomplete dominance, heterozygotes show an intermediate phenotype, illustrated with the red pink white snapdragon example. A cross between pink flowers (Rr) yields a genotype ratio of 1 RR : 2 Rr : 1 rr and a phenotype ratio of 1 red : 2 pink : 1 white, reflecting an in-between phenotype rather than a dominant/recessive dichotomy.

"Incomplete dominance leads to an intermediate phenotype." - Sam

In codominance, both traits are fully expressed. An example with a chicken showing black, white, and black-and-white speckled plumage is used to illustrate how codominant alleles express distinctly in heterozygotes (BB black, WW white, BW black and white). The predicted genotype and phenotype ratios in a cross between a homozygous black and a heterozygous black-white bird are 2 BB to 2 BW, which reduces to 1:1 for both genotype and phenotype outcomes.

Multiple Alleles: Blood Types

Blood type biology is introduced as a classic case of multiple alleles. The ABO system uses IA, IB, and i alleles, with the common simplification that IA and IB are codominant and i is recessive. The video presents a cross where one parent is heterozygous type B and the other heterozygous type A, illustrating a 25 percent chance of type O offspring when combining these alleles. It is noted that blood types can also be positive or negative due to the Rh factor, which adds another layer of complexity that is not covered in this video.

Sex-Linked Traits

The discussion covers genes on sex chromosomes with a focus on X-linked recessive conditions like hemophilia. For a male with the disorder the genotype is written as XhY, while a female with the disease would have the homozygous recessive genotype XhXh. A male with hemophilia and a homozygous dominant female would produce all daughters as carriers and none of the sons would inherit the disease, since sons receive the Y from their father. The section underscores that sex-linked inheritance patterns can differ from autosomal patterns and that the sex of the individual influences genotype interpretation.

Pedigrees

The video ends with a look at pedigrees as a tool for tracking trait inheritance across generations. Shaded shapes typically indicate individuals with the trait of interest, most commonly autosomal recessive or dominant traits. The presenters walk through how to infer genotypes from a pedigree, noting that shading first can help determine whether a trait is autosomal or sex linked. They also discuss the limitations of pedigrees and why real genetic reasoning may require more than a single-gene perspective.

Closing and Additional Resources

The Amoeba Sisters encourage viewers to practice problems, use a sheet of paper for Punnett squares, and rewatch the full content videos in their genetics series for more practice. They highlight that genetics is a complex field with polygenic and pleiotropic traits, epistasis, and epigenetics that extend beyond simple Punnett square problems, and invite viewers to explore real-world examples in the video details.