Inheritance: Autosomal Linkage | A-level Biology | OCR, AQA, Edexcel

Summary

This educational video introduces the concept of autosomal linkage using the example of genetic crosses in the Drosophila fruit fly. It explains the structure of human chromosomes, distinguishing between sex chromosomes and autosomes. The video details how genes on the same chromosome are linked and discusses meiosis and independent assortment in gamete formation. Through a Punnett square analysis of a cross between heterozygous fruit flies with gray bodies and normal wings, it highlights the phenotypic ratios resulting from autosomal linkage, ultimately demonstrating the inheritance patterns of linked genes.

Takeaways

• 😀 Humans have 23 pairs of chromosomes, including 22 pairs of autosomes and 1 pair of sex chromosomes.
• 🧬 Autosomes are non-sex chromosomes that contain genes unrelated to sex determination.
• 🔗 Genes located on the same chromosome are linked and are less likely to assort independently during meiosis.
• 🍃 In normal genetic crosses, genes on separate chromosomes assort independently, leading to more varied offspring combinations.
• 🐞 Drosophila melanogaster (fruit fly) is a common model organism used to study autosomal linkage.
• 🎨 The gene for body color in fruit flies has a dominant gray allele and a recessive black allele.
• ✈️ The wing length gene in fruit flies has a dominant normal wing allele and a recessive vestigial wing allele.
• 📊 A Punnett square can predict the offspring's phenotypes and genotypes when two heterozygous fruit flies are crossed.
• 🧪 Autosomal linkage affects the expected phenotypic ratios, typically resulting in fewer combinations than independent assortment.
• 💡 In the given example, crossing two heterozygous flies results in a phenotypic ratio of 3 gray normal-winged to 1 black vestigial-winged offspring.

Q & A

What is autosomal linkage?

Autosomal linkage refers to genes located on the same chromosome. These genes tend to be inherited together because they are less likely to be separated during meiosis due to their proximity on the chromosome.

How many pairs of chromosomes do humans have, and what are their types?

Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes. The autosomes are involved in non-sex related traits, while the sex chromosomes determine the gender of the individual.

What are autosomes, and how do they differ from sex chromosomes?

Autosomes are chromosomes that are not involved in sex determination. They make up 22 of the 23 pairs of chromosomes. Sex chromosomes, on the other hand, are the 23rd pair and determine the biological sex of an individual.

What does it mean for genes to be linked on a chromosome?

Genes that are located on the same chromosome are considered linked. These genes are less likely to be separated during crossing over in meiosis, which means they are inherited together more frequently.

What is independent assortment in the context of genetic inheritance?

Independent assortment refers to the process during meiosis where genes located on different chromosomes are randomly separated into gametes. This results in a wide variety of genetic combinations in the offspring.

What effect does autosomal linkage have on genetic variation in offspring?

Autosomal linkage reduces genetic variation in offspring because linked genes are inherited together, meaning there are fewer combinations of alleles in the gametes compared to genes on separate chromosomes.

How can we predict the offspring's genotype and phenotype in a dihybrid cross with autosomal linkage?

In a dihybrid cross with autosomal linkage, we predict the offspring's genotype and phenotype using a Punnett square. However, due to autosomal linkage, the number of possible gametes is reduced, and the phenotypic ratio is typically skewed compared to independent assortment.

What is the expected phenotypic ratio in a dihybrid cross between two heterozygous Drosophila fruit flies with autosomal linkage?

In a dihybrid cross between two heterozygous Drosophila fruit flies with autosomal linkage, the expected phenotypic ratio is 3:1. Three of the offspring are expected to have both dominant traits (gray body, normal wings), while one is expected to have both recessive traits (black body, vestigial wings).

How does the crossing over process impact linked genes during meiosis?

During meiosis, crossing over can exchange alleles between homologous chromosomes. However, if genes are closely linked on the same chromosome, they are less likely to be separated during crossing over, leading to the inheritance of linked traits together.

What are the differences in the inheritance patterns of linked genes versus genes on separate chromosomes?

Genes on separate chromosomes follow independent assortment, leading to four possible genetic combinations in the offspring. In contrast, linked genes are less likely to be separated during meiosis, resulting in fewer possible combinations, typically two types of gametes.