13. Genetics 2 – Rules of Inheritance

MIT OpenCourseWare

12 May 202049:00

EducationalLearning

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TLDRThis video script follows a genetics professor teaching students about Gregor Mendel's seminal work discovering laws of inheritance in pea plants. Principles like segregation of alleles into gametes and independent assortment of traits are explained through meiosis. The professor then shifts to discussing Thomas Hunt Morgan's work with fruit flies, which demonstrated sex linkage of traits and led to the discovery that genes are carried on chromosomes. Overall, the script traces major discoveries underpinning genetics from Mendel's plant experiments to breakthroughs linking genes and chromosomes in flies.

Takeaways

😀 Mendel used pea plants to define laws of inheritance because they can self-pollinate and have clear dominant/recessive traits
👀 Pedigrees visualize inheritance patterns across generations, like with PKU disease
👂🏻 Mendel's work went unrecognized until early 1900s when chromosomes provided physical basis
🔬 Yellow peas dominant over green peas in Mendel's monohybrid cross with 3:1 phenotype ratio
🧬 Law of Segregation from meiosis 1 chromosome splitting explains Mendel's 1st law
⏳ Law of Independent Assortment from random meiosis 1 alignment explains dihybrid crosses
🐞 Morgan studied sex-linked white eye color trait in flies controlled by X chromosome
🔀 Reciprocal cross with red-eyed males and white-eyed females posed for next lecture
📈 First genetic map made by undergrad in Morgan lab to be discussed
🤔 Students to ponder reciprocal cross results versus Mendel's pea crosses

Q & A

What enabled Mendel to develop his theory of inheritance?
-Mendel was able to develop his theory of inheritance using pea plants. He took advantage of three key aspects of pea plants: 1) They can self-pollinate or cross-pollinate. 2) They have visible traits with clear dominant/recessive patterns. 3) He used pure breeding lines to start his experiments.
What is Mendel's First Law and how does it relate to meiosis?
-Mendel's First Law states that organisms inherit one gene allele from each parent. This occurs because homologous chromosomes segregate and assort independently during meiosis I, so each gamete has an equal chance of receiving either allele.
Explain Mendel's dihybrid cross experiments.
-In a dihybrid cross, Mendel crossed pea plants that differed in two traits - pea color (yellow vs green) and pea shape (round vs wrinkled). This showed that the two traits assorted independently, giving a 9:3:3:1 phenotypic ratio in the F2 generation. This illustrates Mendel's Second Law of independent assortment.
How did Morgan's work on fruit flies explain sex-linked inheritance?
-Morgan's lab found a white-eyed mutant male fruit fly. By crossing this to a red-eyed female, they showed that the white eye trait was only passed on to male offspring. This sex-linked inheritance is explained by the white gene being located on the X chromosome, of which males only have one copy.
Why did Mendel use pure breeding lines in his experiments?
-Mendel used pure breeding lines, which always give rise to offspring with the same traits, in order to control the genotypes present. Pure breeding lines are homozygous for particular traits, so Mendel could predictably combine selected traits through crosses.
What is the difference between a monohybrid cross and a dihybrid cross?
-A monohybrid cross examines the inheritance of a single trait, while a dihybrid cross looks at two traits. Mendel's monohybrid pea color crosses showed the 3:1 dominant:recessive ratio, while his dihybrid crosses tracking both color and shape showed more complex 9:3:3:1 ratios.
How did Mendel's work lay the foundation for modern genetics?
-Mendel defined key laws of inheritance based on dominant/recessive alleles and independent assortment. Later work linked these abstract 'factors' to physical chromosomes, showing that Mendelian inheritance results from the behavior of chromosomes during meiosis.
Why was the pea plant a good model organism for Mendel's experiments?
-The pea plant was ideal for Mendel because: 1) It can self- or cross-pollinate, 2) It has visible traits that exhibit clear dominant/recessive patterns, and 3) He could generate pure breeding lines to control parent genotypes.
What is the significance of a reciprocal cross?
-A reciprocal cross switches the parental genotypes used in a cross (e.g. red-eyed female x white-eyed male instead of white-eyed male x red-eyed female). This helps determine if both sexes follow the same inheritance pattern.
How did Morgan's fly lab lead to the discovery of sex linkage?
-Morgan's lab found a white-eyed mutant male fly and crossed it to red-eyed females. This showed white eyes were only passed onto sons, demonstrating this trait was sex-linked and connected to the X chromosome.

Outlines

00:00

😀 Mendel's Laws of Inheritance

This paragraph introduces Mendel's laws of inheritance. It discusses how Mendel used pea plants for his studies, taking advantage of their ability to self-pollinate and their clear phenotypes. Mendel started with pure breeding lines to establish his laws. His first law states that organisms have pairs of alleles that segregate equally into gametes, reflecting the segregation of homologous chromosomes in meiosis. His second law is the law of independent assortment, where genes on different chromosomes assort independently, again reflecting the random alignment of homologous chromosomes in meiosis.

05:01

😀 Example of Autosomal Recessive Inheritance in Humans

This paragraph provides an example of autosomal recessive inheritance in humans using the disease phenylketonuria (PKU). It manifests when individuals are homozygous for the defective enzyme allele. The pattern skips generations but appears again with inbreeding, similar to Mendel's observation that recessive traits can disappear and reappear.

10:03

😀 Mendel's Monohybrid Cross Experiments with Pea Color

This paragraph explains Mendel's monohybrid cross experiments involving pea color (yellow versus green). The F1 generation was all yellow, indicating yellow is dominant. The F2 generation exhibited a 3:1 ratio of yellow to green peas. This segregation pattern is explained by the equal probability of gametes receiving the Y or y allele, with only yy individuals exhibiting the green recessive phenotype.

15:04

😀 Mendel's Law of Independent Assortment

This paragraph covers Mendel's law of independent assortment resulting from his dihybrid cross experiments. Different gene pairs segregate independently, reflecting the random alignment of homologous chromosomes in meiosis 1. A 9:3:3:1 phenotypic ratio is observed. The probabilities of different combinations of dominant and recessive traits for two genes can be calculated based on separate monohybrid ratios.

20:05

😀 Morgan's Discovery of Sex Linkage in Fruit Flies

This paragraph shifts to the work of Thomas Hunt Morgan on sex linkage in fruit flies. Morgan identified a white eye mutant male and crossed it to a wildtype red-eyed female. The F1 progeny were all red-eyed. In the F2 generation, white eyes appeared but only in males, similar to human color blindness inheritance. This reflects the mutant allele being on the X chromosome.

25:06

😀 Explaining Sex Linkage through Chromosome Inheritance

This paragraph explains how the sex linkage pattern can be understood by tracing the inheritance of X and Y chromosomes. Females get one X chromosome from each parent while males get their X from the mother and Y from the father. Thus, only males can exhibit recessive X-linked phenotypes like white eyes because they have just one X chromosome copy.

Mindmap

Keywords

💡Mendelian genetics

Mendelian genetics refers to the laws of inheritance first proposed by Gregor Mendel based on his studies of pea plants. Mendel's laws describe how traits are passed from parents to offspring. They include the law of segregation, which states that allele pairs separate randomly during gamete formation, and the law of independent assortment, which states that genes sort independently of each other. Mendelian genetics provides a foundation for understanding genetics and is illustrated through examples like monohybrid and dihybrid crosses in the video.

💡Meiosis

Meiosis is a type of cell division that results in the production of gametes or sex cells. The video explains how meiosis, specifically the separation of homologous chromosome pairs during meiosis I, provides a physical basis for Mendel's laws. The law of segregation reflects the random assortment of chromosomes into gametes during meiosis I. The law of independent assortment reflects the random alignment of chromosome pairs at the metaphase plate during meiosis I.

💡Alleles

Alleles refer to alternative forms of a gene. Mendel used the concepts of dominant and recessive alleles in his experiments, denoting dominant alleles with capital letters (Y) and recessive alleles with lowercase letters (y). The video discusses inheritance patterns based on allele combinations, like how having two recessive alleles leads to diseases like PKU in humans.

💡Inheritance patterns

The video covers different patterns of inheritance for traits including autosomal recessive (PKU), autosomal dominant (pea color in Mendel's experiments), and sex-linked recessive (red-white eye color in flies). These patterns result from the allele combinations offspring inherit from their parents and how those alleles interact to produce phenotypes.

💡Pedigree analysis

Pedigree analysis involves studying inheritance patterns using family trees or pedigrees. The video shows a pedigree indicating autosomal recessive inheritance of PKU. Analyzing pedigrees allows geneticists to determine whether traits are autosomal or sex-linked based on their transmission across generations.

💡Sex linkage

Sex linkage refers to when a trait is carried on a sex chromosome rather than an autosome. The video discusses how the white eye trait in flies is sex-linked and only passed from fathers to sons, similar to color blindness in humans. This is because the gene is located on the X chromosome which males only inherit from their mother.

💡Reciprocal cross

A reciprocal cross is when the parental sexes used in a genetic cross are reversed. The video prompts thinking about how a reciprocal cross between white-eyed female and red-eyed male flies would differ from the initial cross done by Morgan between a white-eyed male and red-eyed female.

💡Monohybrid cross

A monohybrid cross looks at the inheritance pattern of a single trait. Mendel's experiments with pea color (yellow versus green) represent a monohybrid cross. This cross demonstrated the law of segregation and the 3:1 phenotypic ratio seen in the F2 generation.

💡Dihybrid cross

A dihybrid cross analyzes the inheritance of two traits simultaneously. Mendel's experiments crossing pea color and pea shape (round versus wrinkled) represent a dihybrid cross. This demonstrated the law of independent assortment through the 9:3:3:1 F2 phenotypic ratio.

💡Genetic linkage

Genetic linkage refers to genes or alleles that are physically connected on the same chromosome. Linked genes tend to be inherited together. The law of independent assortment only applies when genes are on different chromosomes, as the video explains. Linkage mapping was pioneered in Morgan's fly lab.

Highlights

First significant research finding

Introduction of new theoretical model

Proposal of innovative experimental method

Demonstration of practical real-world applications

Transcripts

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