INHERITANCE - AQA A LEVEL BIOLOGY + EXAM QUESTIONS RUN THROUGH

Introduction

In this video, the instructor introduces the inheritance chapter for A-level biology. The video will cover various topics such as genotype and phenotype, multiple alleles, homozygous and heterozygous alleles, monohybrid and dihybrid crosses, sex linkage, autosomal linkage, epistasis and chi-squared test.

Chapter Overview

This section covers the content that will be covered in the video. The topics include genotype and phenotype, multiple alleles, dominant recessive and codominant alleles, homozygous and heterozygous alleles, monohybrid and dihybrid crosses involving sex linkage autosomal linkage multiple alleles and epistasis.

Genotype vs Phenotype

This section explains what a genotype is (the genetic constitution of an organism) versus what a phenotype is (the expression of the genotype).

• Phenotype is like a feature while genotype is the actual gene or allele that it carries.

• Alleles are variants of a particular gene.

Multiple Alleles

This section explains what multiple alleles are (example: eye color), how they occur (mutations in a gene at different positions), and their impact on organisms.

• Homozygotes have two copies of the same allele while heterozygotes have two different ones.

• Dominant allele only needs one copy for expression while recessive requires two copies.

• Example: Cystic fibrosis allele - capital C denotes normal/dominant allele while lowercase c denotes cystic fibrosis/recessive allele. Two lowercase c means homozygous recessive and affected by cystic fibrosis.

Chi-Squared Test

This section explains what the chi-squared test is and how it is used in genetics.

• The chi-squared test is a statistical method used to determine if there is a significant difference between observed and expected values.

• In genetics, it can be used to determine if observed ratios of offspring from crosses match expected ratios based on Mendelian genetics.

Introduction to Codominant Alleles

In this section, the concept of codominant alleles is introduced. Codominant alleles are equally dominant and expressed in the phenotype.

Understanding Codominant Alleles

• Codominant alleles are equally dominant and expressed in the phenotype.

• Blood type ABO system is an example of codominance.

• Blood group O is recessive while blood groups A and B are codominant.

Predicting Offspring Genotypes with Monohybrid Crosses

This section explains how monohybrid crosses can be used to predict offspring genotypes.

Using Punnett Squares for Monohybrid Crosses

• Monohybrid crosses can be used to predict offspring genotypes.

• Punnett squares can be used to determine possible offspring genotypes.

• Crossing a homozygous dominant individual with a homozygous recessive individual results in 100% heterozygous carriers as offspring.

• Crossing two heterozygotes results in a 3-to-1 ratio of unaffected to affected phenotypes.

Determining Unknown Genotypes with Monohybrid Crosses

This section explains how monohybrid crosses can be used to determine unknown genotypes.

Determining Unknown Genotype with Homozygous Recessive Individual

• Unknown genotype can be determined by crossing it with a homozygous recessive individual.

• If all offspring have dominant phenotype, the unknown genotype is homozygous dominant.

• If half of the offspring have recessive phenotype, the unknown genotype is heterozygous.

Mendelian Genetics

This section covers the basics of Mendelian genetics, including monohybrid and dihybrid crosses, parental genotypes, and phenotypic ratios.

Monohybrid Crosses

• Genes can affect the shape and color of a pea.

• Homozygous dominant individuals have round green peas, while homozygous recessive individuals have wrinkled yellow peas.

• Heterozygotes have one copy of each allele.

• The offspring from a cross between heterozygotes will have a 3:1 ratio of dominant to recessive phenotypes.

Dihybrid Crosses

• In a dihybrid cross, two genes are considered at once.

• The offspring from a cross between heterozygotes for two genes will have a 9:3:3:1 ratio of phenotypes.

Linkage

This section covers autosomal linkage and sex linkage in genetics.

Autosomal Linkage

• Autosomal linkage occurs when two or more genes are located on the same autosome.

• Linked alleles are less likely to be separated during crossing over, resulting in offspring that resemble the parental genotype.

Sex Linkage

• Sex linkage occurs when genes are located on the sex chromosomes.

• Males are more likely to express recessive sex-linked alleles because they only have one X chromosome.

X-Linked Recessive Conditions

This section discusses how males inherit x-linked recessive conditions from their mothers, as they only inherit the y chromosome from their fathers.

Inheritance of X-Linked Recessive Conditions

• Males inherit x-linked recessive conditions from their mothers.

• Females can be carriers of x-linked recessive conditions if they are heterozygous, but they cannot have the disease as they have two X chromosomes and no Y chromosome.

Epistasis

This section explains epistasis, which is an interaction between two genes where one gene masks the expression of another gene in the phenotype. The suppressing gene is called the epistatic gene, while the masked or suppressed gene is called the hypostatic gene.

Dominant and Recessive Epistasis

• Epistasis is an interaction between two genes where one gene masks the expression of another gene in the phenotype.

• The suppressing gene is called the epistatic gene, while the masked or suppressed gene is called the hypostatic gene.

• Dominant epistasis occurs when one or two dominant alleles are present for the epistatic gene. This means that expression of the hypostatic gene is always repressed.

• Recessive epistasis occurs when two copies of the episodic allele are required to mask or suppress expression of a hypostatic allele.

Chi-Squared Test

This section discusses the chi-squared test, which is a statistical test used in biology to determine the probability of an unexpected result being due to chance or being significant.

Understanding the Chi-Squared Test

• The chi-squared test is a statistical test used in biology to determine the probability of an unexpected result being due to chance or being significant.

• The chi-squared test is based on a null hypothesis, which explains that any difference between observed and expected results is due to chance.

• To calculate chi squared, we calculate the sum of observed results minus expected results squared divided by expected results.

• An example of using the chi-squared test involves flipping a coin 50 times and comparing observed versus expected results.

Dihybrid Crosses and Chi-Squared Analysis

In this section, the speaker explains how to calculate expected results in dihybrid crosses and how to use chi-squared analysis to determine if the observed results are significant.

Calculating Expected Results in Dihybrid Crosses

• Monohybrid crosses produce a 3:1 ratio of dominant to recessive phenotypes.

• If we have 100 peas with 80 round and 20 wrinkled, we can calculate expected results by subtracting 75 (expected number of round peas) from 80 (observed number of round peas), which gives us a difference of 5. We do the same for wrinkled peas, giving us a difference of -5.

• We square these differences (25 and 25) and divide them by the expected number of each phenotype (75 for round, 25 for wrinkled). This gives us values of 0.33 for round and 1 for wrinkled.

• Adding these values together gives us our chi-squared value.

Using Chi-Squared Analysis

• Chi-squared analysis is used to determine if observed results are significantly different from expected results.

• The degrees of freedom are calculated as the number of categories minus one. For example, in a coin flipping experiment with two categories (heads and tails), there is one degree of freedom.

• The critical value is the value at a five percent chance that the results are due to chance. It is found on a table using degrees of freedom.

• If the chi-squared value is above the critical value, then the results are significant. If it is below, then they are not significant.

• A p-value of less than 5% means that the results are significant and we can reject the null hypothesis. A p-value of more than 5% means that the results are not significant and we can accept the null hypothesis.

Sex Linkage in Birds

In this section, the speaker explains why recessive sex-linked characteristics are more common in female birds than male birds.

• Females only need one recessive allele for a sex-linked characteristic to be expressed in their phenotype.

• This is because females have two X chromosomes, while males have one X and one Y chromosome.

• If a male has a recessive allele on his X chromosome, he will express the characteristic because there is no dominant allele on his Y chromosome to mask it.

• Females, on the other hand, need two copies of the recessive allele to express the characteristic if they have a dominant allele on their other X chromosome.

Genetics and Feather Production in Chickens

In this section, the speaker discusses a pedigree diagram that shows the results of crosses carried out by a farmer to study feather production in chickens. The speaker explains how a gene on the X chromosome controls the rate of feather production and how different alleles affect feather production.

Pedigree Diagram for Feather Production

• The diagram is a pedigree diagram that shows the main parents, their offspring, and the offspring's offspring.

• The question asks for evidence from the figure that shows that the allele for rapid feather production is recessive.

• Evidence from the figure shows that one must possess or pass on the recessive allele.

Genotypes of Chickens

• Chicken 5 is female with rapid feather production and carries one recessive allele (x lowercase f).

• Chicken 7 is male with slow feather production and could have either x capital f x lower case f or x lower case f x lower case f genotypes.

Genetics: Feather Production

In this section, the speaker discusses genetics and feather production in chickens. They explain how to write genotypes for different traits and how to determine the genotypes of parents based on their offspring.

Writing Genotypes

• The letter "f" is often used to denote feather production in chickens.

• To get a mark, you can use either "x lowercase f y" or "x lowercase f x lowercase f".

• Any letter in the alphabet can be used instead of "f", but it's preferred to use "f" since it stands for feather.

Determining Parent Genotypes

• Male chickens carry two X chromosomes, so males with rapid feather production are homozygous and recessive.

• For a male chicken to have rapid feather production, both parents must carry the recessive allele.

• The genotypes of the parents can be determined based on their offspring. If two offspring have rapid feather production and two have slow feather production, then the father is heterozygous (X capital F X lowercase f) and the mother is X lowercase f Y.

Conclusion

In this video, we learned about genetics and how they relate to feather production in chickens. We also learned how to write genotypes for different traits and how to determine parent genotypes based on their offspring.