Inheritance (DP IB Biology)

An allele is a version of a gene, e.g. the gene that codes for eye colour may give rise to blue or brown eyes.

A genotype is the combination of alleles that an individual organism inherits.

Homozygous is when the genotype has two of the same allele.

Heterozygous is when the genotype has two different alleles.

The phenotype is the observable characteristics of an organism. This can be visibly observed (e.g. eye colour) or tested for (e.g. blood type).

False.

The phenotype is determined by the genotype, the environment, or an interaction between both.

Codominance is when both alleles are expressed to an equal extent in the phenotype.

Incomplete dominance is when two alleles are partially expressed, leading to a phenotype which is a blend or intermediate between the two.

False.

A dominant allele only needs to be inherited from one parent to be expressed in the phenotype.

False.

Inheritance of flower colour in the marvel of Peru plant is an example of incomplete dominance.

Phenotypic plasticity is the idea that although genotype remains fixed throughout an organism's lifetime, the way that the phenotype is expressed can vary due to environmental influences.

False.

Phenotypic plasticity refers to changes in phenotype expression, while the genotype remains fixed throughout an organism's lifetime.

A Single Nucleotide Polymorphism (SNP) is the exact position where bases differ between alleles.

Phenylketonuria (PKU) is an inherited condition caused by a recessive allele on an autosome, leading to a build-up of the amino acid phenylalanine in the body.

PKU follows an autosomal recessive inheritance pattern, requiring two copies of the recessive allele for the condition to manifest.

Multiple alleles refer to the existence of more than two possible allelic forms of a gene within a population.

True.

Many genes have more than two alleles, but a diploid individual will still only inherit two of the possible alleles.

Codominance in blood groups means that both the I^A and I^B alleles are expressed equally when present together, resulting in AB blood type.

The alleles for blood group show the following relationship:

• I^A and I^B are codominant

• I^A and I^B are dominant to the recessive allele, i

Their child has a 50% chance of having type A blood and a 50% chance of type B blood.

This can be explained in the Punnett grid below:

Sex chromosomes are the chromosomes that determine the biological sex of an individual; in humans they are the X and Y chromosomes.

In biological females the sex chromosomes are XX.

In biological males the sex chromosomes are XY.

False.

Biological sex is determined by an entire chromosome pair (XX for females, XY for males), not by a single gene.

True.

The father determines the biological sex of the child because he can pass on either an X or a Y chromosome.

The probability of a child inheriting two X chromosomes is 1/2 or 50 %.

False.

The X chromosome carries around 16 times more genes than the Y chromosome.

Sex-linked disorders are caused by genes that are only present on one sex chromosome and not the other.

The three possible phenotypes for females regarding sex-linked disorders are: normal, carrier, and has the disease.

Haemophilia is a sex-linked disease caused by a recessive allele on the X chromosome, resulting in a lack of factor VIII protein needed for blood clotting.

Sex-linked alleles should be notated using an upper case 'X' or 'Y' for the chromosome, with superscript letters representing the allele. For example, X^FX^f represents a heterozygous female carrier for haemophilia.

A haemophiliac father and a carrier mother have a 50% chance of having a child with haemophilia.

This can be shown in the Punnett grid below:

A pedigree chart is a family diagram used to trace the pattern of inheritance of a specific characteristic, usually a disease, through generations of a family.

In a pedigree chart, males are represented by squares and females are represented by circles.

Shading or cross-hatching in a pedigree chart typically indicates individuals that are affected by a genetic-caused trait.

False.

In a pedigree chart, horizontal lines between males and females show that they have produced children together.

If a genetic condition skips a generation in a pedigree chart, it typically indicates that the condition is caused by a recessive allele.

False.

In a pedigree chart, Roman numerals are used to indicate generations.

Inductive reasoning is making generalised conclusions based on specific evidence taken from a small sample, such as observing a pattern in a pedigree chart to determine if a condition is sex-linked.

Deductive reasoning is making specific deductions about something unknown based on known evidence, such as deducing that a condition is caused by a recessive allele when two non-affected parents have an affected child.

Discrete variation is variation that falls into two or more clear-cut categories with no overlap or in-between categories.

Continuous variation occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics, with a range of values existing between two extremes.

A polygenic characteristic is a trait determined by more than one gene.

False.

Height in humans is an example of continuous variation, determined by many genetic and environmental factors.

In continuous variation, phenotype = genotype + environment.

Box plots, also known as box-and-whisker diagrams, are used when we are interested in splitting data up into quartiles and seeing what is happening at the low, middle, and high points of a dataset.

The five values needed to draw a box plot are: lowest data value, first quartile, median, third quartile, and highest data value.

The interquartile range in a box plot represents the middle 50% of the data.

Outliers are data points that are more than 1.5 times the interquartile range above the third quartile or below the first quartile.

False.

The median in a box plot will not necessarily be in the middle of the box.

True.

Independent assortment occurs because unlinked genes segregate independently during meiosis.

Independent assortment is the principle that alleles for different traits segregate independently of one another during meiosis if they are unlinked.

In dihybrid crosses involving unlinked autosomal genes, the typical phenotypic ratio expected in the offspring is 9:3:3:1

Punnett grids can be used for predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes.

False:

A 1:1:1:1 ratio in dihybrid crosses is expected when a heterozygous individual is crossed with a homozygous recessive individual for both traits.

False.

Mendel's second law applies to genes on different chromosomes or far enough apart on the same chromosome that they are not linked.

In statistical testing, a p-value less than 0.05/5% indicates that the observed results are statistically significant.

The null hypothesis is the assumption that there is no significant difference between observed and expected ratios, suggesting any difference is due to chance.

A chi-square test compares observed results (actual data) to expected results (predicted ratios) to determine if deviations are due to chance.

False.

The null hypothesis would be rejected, indicating a significant difference between observed and expected results.

The F2 generation of offspring is typically used as the sample in chi-squared tests for dihybrid crosses.

The alternative hypothesis is an alternative to the null hypothesis and suggests that there is a statistically significant difference between observed and expected outcomes.

Replication provides more data, helping ensure that results are reliable and that any conclusions about statistical significance are more accurate.

True.

A gene's locus refers to its specific position on a chromosome.

A polypeptide product is the chain of amino acids produced by the translation of mRNA, which corresponds to the protein encoded by a gene.

True.

Genes that are close in proximity on the same chromosome can sometimes be inherited as a single unit due to genetic linkage.

False.

The first number of the locus location code represents the chromosome number. These two genes are found on different chromosomes and so are not linked.

False.

In genetic diagrams involving linked genes, alleles should be shown alongside vertical lines representing homologous chromosomes.

Alleles of linked genes sometimes fail to assort independently because they are close together on the same chromosome, and therefore, they tend to be inherited together unless crossing over separates them.

Genes located close together on the same chromosome are known as linked genes.

Autosomal gene linkage refers to the phenomenon where genes located on the same non-sex (autosomal) chromosome tend to be inherited together.

True.

Crossing over can separate linked genes, allowing for independent assortment.

Recombinants are offspring with a combination of alleles that differs from that of either parent, resulting from crossing over during meiosis.

False.

In a test cross between a heterozygous individual for two linked genes and a homozygous recessive individual for both genes, the presence of recombinant offspring suggests that crossing over has occurred.

This is because recombinants are offspring with a combination of alleles that differs from that of either parent.

Recombinants can be identified by comparing offspring phenotypes and genotypes to see if they display new allele combinations not present in either parent.

False.

Recombinants are offspring that display a new combination of traits different from either parent.

In a test cross involving two genes, the recombinant offspring result from crossing over during meiosis.

False.

In a cross between a heterozygous individual and a homozygous recessive individual, some offspring will show parental phenotypes if the genes are linked. Some offspring may be recombinants if crossing over occurs.