IB DP Biology Inheritance Study Notes
IB DP Biology Inheritance Study Notes
IB DP Biology Inheritance Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of
- • What patterns of inheritance exist in plants and animals?
• What is the molecular basis of inheritance patterns?
Standard level and higher level: 5 hours
Additional higher level: 3 hours
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
D3.2.1—Production of haploid gametes in parents and their fusion to form a diploid zygote as the means of inheritance
Parents pass genes to offspring through gametes. Gametes are haploid, meaning they contain one chromosome of each type. When a male and female gamete fuse, their nuclei join, doubling the chromosome number to form a diploid zygote. To produce haploid gametes, parents undergo meiosis, halving the chromosome number in their body cells. This ensures that offspring inherit an equal genetic contribution from both parents. The human life cycle follows this pattern, with 46 chromosomes in body cells and 23 in gametes.
D3.2.2—Methods for conducting genetic crosses in flowering plants
To study patterns of inheritance, scientists cross different varieties of flowering plants. Pollen from the anther of one plant (male parent) is transferred to the stigma of another plant (female parent). This cross-pollination results in fertilization and the production of seeds. The offspring from this cross are called the F1 generation.
To prevent self-pollination, the anthers of the flower can be removed before they release pollen. The flower is then enclosed in a bag to prevent pollen from other plants from reaching the stigma.
Gregor Mendel, a pioneer in genetics, used pea plants to study inheritance patterns. He crossed different varieties of pea plants and analyzed the traits of the offspring. By carefully counting the number of plants with different traits, Mendel was able to discover the basic principles of inheritance.
D3.2.3—Genotype as the combination of alleles inherited by an organism
Different versions of a gene, called alleles, can exist. They may differ slightly or significantly in their DNA sequence. Humans and other diploid organisms inherit two alleles for each gene, one from each parent. Genotypes refer to the combination of alleles an individual possesses.
If an individual has two identical alleles for a gene (e.g., DD or dd), they are homozygous. If they have two different alleles (e.g., Dd), they are heterozygous.
The genotype of an individual determines which alleles they can pass on to their offspring. For example, an individual with the genotype DD can only produce gametes with the D allele, while an individual with the genotype Dd can produce gametes with either the D or d allele.
D3.2.4—Phenotype as the observable traits of an organism resulting from genotype and environmental factors
The phenotype of an organism is its observable traits or characteristics. These traits can be structural, such as hair color, or functional, such as the ability to taste certain flavors. Most phenotypic traits are influenced by both genotype (genetic makeup) and environmental factors.
Some traits are determined solely by genotype, such as ABO blood groups and certain genetic disorders. Other traits, like height and skin color, are influenced by both genes and environmental factors. Environmental factors such as nutrition, sunlight exposure, and temperature can affect how genes are expressed.
D3.2.5—Effects of dominant and recessive alleles on phenotype
Gregor Mendel, considered the father of genetics, used pea plants to study inheritance patterns. He observed that traits like flower color and plant height were inherited in distinct patterns, not blending as previously thought.
Mendel’s experiments revealed the concept of dominant and recessive alleles. Dominant alleles mask the expression of recessive alleles. For example, in pea plants, the allele for tallness (T) is dominant over the allele for dwarfness (t). A plant with the genotype Tt will be tall because the dominant T allele masks the recessive t allele.
Mendel’s work established the foundation of modern genetics, demonstrating how traits are passed from one generation to the next through the inheritance of alleles.
D3.2.6—Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression
Phenotypic plasticity allows organisms to adapt to their environment by varying their gene expression and developing traits suited to their surroundings. This adaptation is reversible as it involves switching genes on or off, without altering the underlying genetic sequence. It is particularly useful in heterogeneous environments where conditions can change.
An example of phenotypic plasticity is the tanning response in humans. When exposed to sunlight, the skin produces more melanin, a pigment that protects the skin from UV radiation. This change in skin color is reversible, and the skin will gradually become paler if sunlight exposure decreases.
In some cases, phenotypic plasticity can involve a switch between two or more distinct forms. These changes may be irreversible within an individual’s lifetime.
D3.2.7—Phenylketonuria as an example of a human disease due to a recessive allele
A genetic disease is caused by a recessive allele of a gene. Individuals with two copies of the recessive allele will develop the disease, while those with one recessive and one dominant allele will be carriers. Genetic diseases often appear unexpectedly, as both parents can be carriers without showing symptoms. The probability of two carriers having a child with the disease is 25%.
Phenylketonuria (PKU) is a genetic disease caused by a recessive allele that impairs the production of the enzyme phenylalanine hydroxylase. This enzyme converts phenylalanine into tyrosine. Individuals with PKU cannot properly metabolize phenylalanine, leading to its accumulation in the body. Excess phenylalanine can impair brain development, leading to intellectual disability.
D3.2.8—Single-nucleotide polymorphisms and multiple alleles in gene pools
A gene pool consists of all the different alleles present in a population. Each individual inherits a combination of alleles from the gene pool, and evolution involves changes in the frequency of these alleles over time.
Single-nucleotide polymorphisms (SNPs) are variations in a single nucleotide within a gene. These variations can lead to the formation of different alleles, increasing the diversity within a gene pool. For example, the S-gene in apples has multiple alleles, preventing self-pollination and promoting genetic diversity.
The concept of multiple alleles within a gene pool highlights the complexity of genetic variation and the potential for evolution. By understanding the genetic diversity within populations, scientists can gain insights into the mechanisms of adaptation and the evolution of new species.
D3.2.9—ABO blood groups as an example of multiple alleles
The ABO blood group system in humans is an example of multiple alleles. A single gene determines the ABO blood group, with three possible alleles: IA, IB, and i. These alleles result in six possible genotypes and four blood groups: A, B, AB, and O.
The IA and IB alleles are codominant, meaning that both alleles are expressed in an individual with the AB genotype. The i allele is recessive to both IA and IB.
Understanding the ABO blood group system is crucial for blood transfusions. Donating the wrong blood type can lead to serious complications.
D3.2.10—Incomplete dominance and codominance
In some cases, neither allele in a gene pair is completely dominant over the other. This leads to incomplete dominance, where the heterozygous phenotype is intermediate between the two homozygous phenotypes.
An example of incomplete dominance is flower color in the plant Mirabilis jalapa. When a red-flowered plant is crossed with a white-flowered plant, the offspring have pink flowers. This intermediate phenotype is due to the blending of the red and white pigments.
Another example is the coat color in Icelandic horses. The alleles for chestnut and white coat color show incomplete dominance. Horses with the heterozygous genotype have a cream or palomino coat color, which is an intermediate between chestnut and white.
Incomplete dominance demonstrates that gene expression is not always straightforward and that the phenotype can be influenced by the interaction of multiple alleles.
D3.2.11—Sex determination in humans and inheritance of genes on sex chromosomes
Sex in humans is determined by the 23rd pair of chromosomes. Females typically have two X chromosomes, while males have one X and one Y chromosome. The Y chromosome contains genes that initiate male development.
Sex chromosome abnormalities can lead to conditions like Klinefelter syndrome (XXY) and Turner syndrome (X). These conditions demonstrate the role of sex chromosomes in determining sex development.
Early in development, gonads have the potential to develop into either testes or ovaries. The presence of a Y chromosome triggers the development of testes, while its absence leads to the development of ovaries.
D3.2.12—Haemophilia as an example of a sex-linked genetic disorder
Haemophilia is a sex-linked genetic disorder caused by a recessive allele on the X chromosome. Males are more likely to be affected because they only have one X chromosome. Females can be carriers of the haemophilia allele but usually do not exhibit symptoms.
People with haemophilia lack or have a defective form of Factor VIII, a clotting factor essential for blood clotting. This leads to prolonged bleeding, especially in response to injuries or surgery. Treatment involves regular infusions of Factor VIII to help the blood clot properly.
The low frequency of haemophilia in females is due to the fact that both X chromosomes would need to carry the recessive allele. This is rare, as fathers with haemophilia would have to pass on the defective allele to their daughters, and the mother would also need to be a carrier.
D3.2.13—Pedigree charts to deduce patterns of inheritance of genetic disorders
Pedigree charts are used to track the inheritance patterns of genetic disorders within families. By analyzing the occurrence of the disorder across generations, researchers can determine whether the trait is dominant or recessive, autosomal or sex-linked.
Key symbols used in pedigree charts:
- Squares represent males.
- Circles represent females.
- Shaded shapes indicate individuals affected by the disorder.
- Horizontal lines connect parents.
- Vertical lines connect parents to offspring.
By studying pedigree charts, geneticists can identify patterns of inheritance and predict the risk of individuals inheriting a particular genetic disorder. This information is crucial for genetic counseling and family planning.
D3.2.14—Continuous variation due to polygenic inheritance and/or environmental factors
Continuous variation refers to traits that have a range of possible values, with no distinct categories. Examples include height, weight, and skin color. This type of variation is often influenced by multiple genes (polygenic inheritance) and environmental factors.
Discrete variation, on the other hand, refers to traits with distinct categories, with no intermediates. Examples include blood type and eye color. These traits are typically determined by a single gene or a few genes with limited alleles.
Understanding the difference between continuous and discrete variation is important for understanding how traits are inherited and how they can be influenced by both genetics and environmental factors.
D3.2.15—Box-and-whisker plots to represent data for a continuous variable such as student height
Box-and-whisker plots are a visual representation of data that shows the minimum value, lower quartile, median, upper quartile, and maximum value (excluding outliers). They provide a quick and easy way to understand the spread and distribution of data.
Key features of box-and-whisker plots:
- Box: Represents the interquartile range (IQR), which contains 50% of the data.
- Whiskers: Extend from the box to the minimum and maximum values, excluding outliers.
- Outliers: Data points that fall outside the whiskers are considered outliers.
Interpreting box-and-whisker plots:
- Central Tendency: The median represents the middle value of the data.
- Spread: The IQR shows the range of the middle 50% of the data.
- Outliers: Outliers can indicate unusual or extreme data points.
- Skewness: The shape of the box and whiskers can suggest whether the data is skewed (asymmetric) or symmetric.
Box-and-whisker plots are a valuable tool for comparing data sets, identifying outliers, and understanding the distribution of data.
D3.2.16—Segregation and independent assortment of unlinked genes in meiosis
Segregation is the separation of alleles of a gene during meiosis. In a diploid cell, there are two alleles of each gene. During gamete formation, these alleles separate, so each gamete receives only one allele. For example, a person with the blood group AB (genotype IAIB) will produce gametes containing either the IA or IB allele.
Independent assortment is the random distribution of different gene pairs into gametes. The inheritance of one gene does not affect the inheritance of another gene. For example, a person with blood group AB and sickle cell anemia (genotype IAIB HSHs) will produce gametes with all possible combinations of alleles: IAHA, IAHs, IBHA, and IBHs.
Both segregation and independent assortment occur during meiosis, specifically during anaphase I, when homologous chromosomes separate. This process generates genetic variation in offspring, as different combinations of alleles are inherited from each parent.
D3.2.17—Punnett grids for predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes
Dihybrid crosses involve studying the inheritance of two genes simultaneously. Mendel performed dihybrid crosses with pea plants, examining traits like seed shape and color.
When Mendel crossed pure-breeding round yellow peas with pure-breeding wrinkled green peas, all F1 offspring had round yellow seeds. This indicated that the alleles for round and yellow were dominant.
When the F1 plants were allowed to self-pollinate, the F2 generation exhibited a phenotypic ratio of 9:3:3:1. This ratio indicates that the two genes are inherited independently, meaning the inheritance of one gene does not affect the inheritance of the other. This is known as the law of independent assortment.
The Punnett square is a useful tool for predicting the genotypic and phenotypic ratios of offspring in dihybrid crosses. By analyzing the possible combinations of gametes, we can determine the probability of different genotypes and phenotypes.
D3.2.18—Loci of human genes and their polypeptide products
The human genome contains approximately 20,000 genes, each located at a specific position on one of the 22 autosomes or the two sex chromosomes (X and Y). These genes code for the amino acid sequences of polypeptides, which are the building blocks of proteins.
The locus of a gene refers to its specific position on a chromosome. For example, the gene responsible for phenylketonuria (PKU) is located on chromosome 12.
By using databases like OMIM and Ensembl, researchers can access information about genes, their locations, and their associated diseases. This information is crucial for understanding genetic disorders and developing potential treatments.
D3.2.19—Autosomal gene linkage
Genes located close together on the same chromosome tend to be inherited together, a phenomenon known as gene linkage. This occurs because these genes are physically linked on the DNA molecule and do not assort independently during meiosis.
An example of gene linkage was first observed in the plant Lathyrus odoratus. When a purple-flowered, long-pollen plant was crossed with a red-flowered, round-pollen plant, the F1 generation all had purple flowers and long pollen. However, when the F1 plants were self-pollinated, the F2 generation did not show the expected 9:3:3:1 ratio. Instead, there were more parental combinations (purple long and red round) than expected.
This deviation from the expected ratio is due to gene linkage. The genes for flower color and pollen shape are located close together on the same chromosome, so they tend to be inherited together. However, crossing over during meiosis can occasionally separate these linked genes, producing recombinant offspring with new combinations of alleles. These recombinant offspring are less common than the parental combinations.
Gene linkage is an important concept in genetics because it affects the inheritance patterns of traits and can be used to map genes on chromosomes. By studying the frequency of recombination between linked genes, scientists can estimate the distance between them on the chromosome.
D3.2.20—Recombinants in crosses involving two linked or unlinked genes
Recombinants are individuals with a different combination of alleles and traits from either parent. They arise from genetic recombination during meiosis.
Random orientation of bivalents and crossing over are two key processes that contribute to genetic recombination. Random orientation results in new combinations of unlinked genes, while crossing over exchanges genetic material between homologous chromosomes, producing new combinations of linked genes.
The frequency of recombination between two genes can be measured by crossing heterozygous individuals with homozygous recessive individuals. The frequency of recombinant offspring is directly related to the distance between the two genes on the chromosome. Genes that are closer together are less likely to recombine, while genes that are farther apart are more likely to recombine.
By studying recombination frequencies, scientists can create genetic maps that show the relative positions of genes on chromosomes.
D3.2.21—Use of a chi-squared test on data from dihybrid crosses
The chi-squared test is a statistical method used to determine if observed data fits a predicted distribution. In genetics, it’s used to assess if observed phenotypic ratios in dihybrid crosses match the expected Mendelian ratios. The test calculates the difference between observed and expected frequencies, and a high chi-squared value suggests a significant deviation from the expected ratio.