IBDP BIOLOGY Unit 3 Genetics – Inheritance (SL & HL)

D3.2.1 Production of Haploid Gametes and Formation of Diploid Zygote

  • Inheritance: Traits passed from parent to offspring during reproduction.
  • DNA: The molecule of inheritance for all living organisms.
  • Chromosomes: DNA is inherited through the passing of chromosomes between generations.
  • Eukaryotic Organisms: Produce haploid gametes in a sexual life cycle.
  • Gametes: Haploid cells with one chromosome of each type.
  • Fertilization: Fusion of male and female gametes to form a zygote.
  • Zygote: Diploid cell with two chromosomes of each type.
  • Chromosome Number Restoration: Uniting two gametes restores the full chromosome number each generation.
  • Human Chromosomes: At fertilization, two haploid gametes combine to form a zygote with 2n = 46 chromosomes.
  • Gametes Definition: Haploid cells that fuse together at fertilization to produce the single-cell zygote.
  • Zygote Definition: The diploid cell produced at fertilization when gametes fuse.
  • Male Gametes: Small, self-propelling cells called sperm.
  • Female Gametes: Large, passively moved cells called egg (or ovum).

D3.2.2 Methods for Conducting Genetic Crosses in Flowering Plants

  • Gregor Mendel (1822-1884): An Austrian monk who studied heredity in plants.
  • Heredity: The passing down of traits from one generation to another.
  • Blending Inheritance: The widely accepted hypothesis that traits of parents blended in the offspring.
    • Example: A white flowered plant and a purple flowered plant would produce an intermediate, or light purple flowered plant.
  • Mendel's Experiments: Mendel carried out a series of experiments on the common pea plant between 1856 and 1863 to test blending inheritance.
  • Plant Reproduction: Reproduction in flowering plants is sexual.
  • Male Gametes: Found in pollen grains on the flower anther.
  • Female Gametes: Inside the ovules of the flower ovary.
  • Pollination: Pollen is transferred from the anthers of one plant to the stigma of another plant.
  • Genetic Crosses Use: Used to breed new varieties of crops or ornamental plants and to demonstrate patterns of inheritance.
  • Cross-Pollination: When one plant fertilizes another plant of the same species.
  • Mendel's Method: Mendel purposefully cross-pollinated pea plants based on their different features.
  • Establishing Pure Lines: Mendel first established pea plant populations with two different features (e.g., tall vs. short height), breeding them until they always produced offspring identical to the parent.
  • Crossing Plants: After establishing pure lines, he crossed the plants with each other to observe how the offspring inherited the traits.
  • P Generation: The parent plants in the experiments are referred to as the P (for parent) generation.
  • F1 Generation: The resultant plants from crossing the P generation. All F1 plants were identical in shape to one of the parents.
  • Dominant and Recessive Phenotypes: Mendel concluded that one of the phenotypes masked the expression of the other phenotype.
    • Dominant Phenotype: The expressed phenotype of the F1.
    • Recessive Phenotype: The repressed phenotype.
  • Self-Fertilization: Pollen grains from anthers on one plant are transferred to stigmas of flowers on the same plant.
  • F2 Generation: Results from self-pollination of the F1 generation. The recessive trait reappeared in a 3:1 ratio (dominant to recessive).
  • Particulate Inheritance Hypothesis: Mendel concluded that phenotypes were determined by the combination of discrete heritable units.
  • Alleles: Alternate versions of a gene.

Punnett Grids

  • Reginald Crundall Punnett (1875-1967): A British geneticist who devised the Punnett Square.
  • Punnett Square: A visual representation of Mendelian inheritance.
  • Steps for Completing a Punnett Grid:
    1. Determine gene, alleles, and parental diploid genotypes.
    2. Determine the unique gametes from each parent that result from segregation of alleles during meiosis.
    3. Draw a Punnett grid using unique gametes only.
    4. Fill in the Punnett grid with the possible genotypes of the offspring.
    5. Summarize possible genotypes and phenotypes of offspring, with expected ratios.
  • Punnett Grid Variation: A Punnett Grid is not always a "square"; depending on the genotypes of the parents, a "rectangle" might be more appropriate.
  • Homozygous: TT
  • Heterozygous: Tt
  • TT x Tt
  • AA parent - all gametes will have “A”, so there is ONE type of unique gamete
  • Aa parent - gametes can have either “A” or “a”, so there are TWO types of unique gametes
  • aa parent - all gametes will have “a”, so there is ONE type of unique gamete
  • Examples of Punnett Grids based on gamete types:
    • AA x aa --> 1 gamete type X 1 gamete type = 1 box Punnett
    • AA x Aa --> 1 gamete type X 2 gamete types = 2 box Punnett
    • Aa x Aa --> 2 gamete types X 2 gamete types = 4 box Punnett
  • Each box of the Punnett grid represents an equal probability of a genetic outcome for an offspring of a cross between two parents.
    • Offspring Genotypes = 1/2 TT, 1/2 Tt
    • Offspring Phenotypes = 4/4 tall plants

D3.2.3 Genotype as the Combination of Alleles Inherited by an Organism

  • Gene: A sequence of DNA nucleotides that codes for an RNA or protein product. Genes are located on chromosomes.
  • Transcription: The synthesis of RNA using the gene as a template.
  • Translation: The synthesis of a polypeptide from mRNA.
  • Genetic Code: The sequence of nitrogenous bases in nucleic acids forms a code that stores information used to make proteins in gene expression.
  • Alleles: Different versions of the same gene.
    • Because they are the same gene, alleles code for the same type of protein
    • Alleles of a gene may be different by a single base in the sequence of the gene or by large sections
    • New alleles are generated by mutation which can form single-nucleotide polymorphisms
    • Alleles are often dominant or recessive to other alleles
  • Diploid Organisms: Have two copies of each chromosome, one inherited from each parent. Therefore, they have two copies of each gene.
  • Genotype: The combinations of alleles for the two copies of a gene within a diploid cell.
    • Homozygous: The two copies of the gene are of the same allele (AA or aa).
    • Heterozygous: The two copies of the gene are different alleles (Aa).

D3.2.4 Phenotype as Observable Traits

  • Phenotype: The observable traits or characteristics of an individual including:
    • Morphology (physical structures)
    • Developmental processes
    • Biochemical processes
    • Physiology
    • Behaviors
  • Influence on Phenotype: Both genes and the environment influence phenotype.
  • Examples of Human Phenotypes:
    • Determined by Genotype Only:
      • ABO blood group
      • PKU disease
      • Hemophilia disease
    • Determined by Environment Only:
      • Scars
      • Body art (piercings and tattoos)
      • Language spoken
    • Determined by Interaction of Genotype with the Environment:
      • Height
      • Development of Type II diabetes
      • Tanning of skin
      • Development of coronary heart disease
  • Gene expression is the mechanism by which information in genes has effects on the phenotype

D3.2.5 Effects of Dominant and Recessive Alleles on Phenotype

  • Humans as Diploid Organisms: Humans have two copies of each chromosome and therefore two copies of each gene (on autosomes).
  • Sex Chromosomes: People who are chromosomally male will have only one copy of any gene on the X and Y chromosomes.
  • Karyogram: A karyogram depicts the chromosomes of an organism, arranged in pairs.
  • Gene Location:
    • MR1C gene ("red hair gene") is on chromosome 16.
    • FGFR3 gene ("dwarfism gene") is on chromosome 4.
  • Phenotype Influence: Phenotypes depend on which two versions of the gene (alleles) are present in the genotype.
  • Dominant vs. Recessive:
    • Alleles described as "dominant" or "recessive" over other alleles.
  • Dominant Allele:
    • Definition: A variation of a gene that will produce a certain phenotype, even if the individual only has one copy.
    • Notation: Capital letter.
    • Cause: Encodes for a functioning protein. One copy of the allele is sufficient to make enough of the protein.
    • Example: Dark hair (R) is dominant over blonde or red hair (r). Even one copy of the dark hair allele will code for enough melanin protein for the hair to look dark.
  • Recessive Allele:
    • Definition: A variation of a gene that must be homozygous when inherited in order to be expressed in the phenotype.
    • Notation: Lowercase letter.
    • Cause #1: Codes for a non-functioning protein.
      • Example: A protein called MC1R functions to get rid of red pigment. The dominant allele of the MC1R gene (R) codes for a functional protein whereas the recessive allele (r) codes for nonfunctioning protein, making hair red.
    • Cause #2: The recessive allele is the normal one and the dominant allele is a mutated version, and we need two good copies of the allele for normal function.
      • Example: Achondroplasia, or dwarfism, is caused by a broken version of the FGFR3 gene. The normal job of the FGFR3 gene (allele a) is to code for a protein that prevents bone growth. The broken version of FGFR3 (allele A) is hyperactive -- it tells bones to stop growing even when they should be growing. The hyperactive FGF3 protein causes a person's bones to be much shorter than normal. Even if a person has a normal copy of FGFR3 (a), the broken version sends a signal that's just too strong (making A dominant).

D3.2.6 Phenotypic Plasticity

  • Phenotypic Plasticity Defined: Ability of an organism to express different phenotypes depending on the biotic or abiotic environment.
  • Mechanism: Involves regulatory genes that switch on structural genes given the appropriate stimulus.
  • Environmental Impact: The environment impacts gene expression.
  • Examples:
    • Human hair and skin color impacted by exposure to sunlight and high temperatures.
    • Pigments in the fur of Himalayan rabbits (Oryctolagus cuniculus) are regulated by temperature.
  • Himalayan Rabbit Example:
    • Gene C controls fur pigmentation.
    • Active between 15 and 25°C; inactive at higher temperatures.
    • Warm weather: No pigment is produced, and the fur is white.
    • Low temperatures: Gene C becomes active in the rabbit's colder extremities (ears, nose, feet) and produces a black pigment.

D3.2.7 Phenylketonuria as a Human Disease Due to a Recessive Allele

  • Genetic Disease Defined: An illness caused in whole or in part by a change in the DNA sequence away from the normal sequence.
  • Single-Gene Diseases: A mutation in just one gene is responsible for disease.
    • Normal vs. Disease-Causing Alleles: There are "normal" alleles of the gene and "disease causing" alleles.
  • Dominant vs. Recessive Genetic Diseases:
    • Dominant: If a person has one dominant allele then they themselves will develop the disease. Example: Huntington's disease
    • Recessive: The disease only develops in individuals that do not have the dominant allele of the gene. Examples: Cystic fibrosis, Phenylketonuria
  • Autosomal vs. Sex-Linked Genetic Diseases:
    • Autosomal: The gene is located on an autosome, so males and females are equally affected.
    • Sex-linked: The gene is located on a sex chromosome, resulting in a different pattern of inheritance in males and females. Examples: Red-green colorblindness, Hemophilia
  • Rarity of Genetic Diseases: Genetic diseases are relatively rare in a population.
  • Recessive Alleles: Most genetic diseases are caused by a recessive allele of a gene.
    • Carriers: Both parents of a child with the disease may be carriers. Carriers have one recessive allele for the genetic disease and one dominant normal allele. They will not show symptoms of the disease but can pass on the recessive allele to their offspring.
  • Phenylketonuria (PKU):
    • Inheritance Pattern: Autosomal recessive.
    • Parents of an individual with PKU each carry one copy of the mutated gene but typically do not show signs and symptoms because they have a normal allele.
    • Probability: There is a 1 in 4 chance the offspring of two carriers will have PKU disease.
    • Cause: Mutations in the PAH gene, located on chromosome 12.
    • Normal Allele: Codes for functioning phenylalanine hydroxylase, which converts the amino acid phenylalanine to another amino acid called tyrosine.
    • Disease Allele: Codes for a non-functioning phenylalanine hydroxylase. Phenylalanine accumulates, and tyrosine becomes deficient.
    • Effect: Excessive phenylalanine levels can cause brain damage.
    • Treatment: The treatment consists of a diet containing little or no phenylalanine.

D3.2.8 Single Nucleotide Polymorphisms and Multiple Alleles in Gene Pools

  • Single Nucleotide Polymorphism (SNP): A variation at a single position in a DNA sequence among individuals.
  • DNA Code: Formed from a chain of four nucleotide bases: A, C, G, and T.
  • Alleles: If a SNP occurs within a gene, then the gene is described as having more than one allele.
  • Formation: SNPs are formed through point mutations to the DNA.
    • Point Mutation: Occurs when a single base pair is added, deleted, or changed.
  • Variations: SNPs result in many different alleles (versions) of each gene in the entire population of a species; there is no limit to the number of alleles of a gene in the population. Each minor change (mutation) in the DNA sequence of a gene can result in a new allele!
  • High Number of Alleles: In the human genome, genes involved with the immune response have the greatest number of alleles. For example, there are more than 32,000 known alleles of the Major Histocompatibility Complex (MHC) genes.
  • MHC Genes: Code for cell surface glycoproteins that differentiate between self and non-self in vertebrate animals.
  • MHC Function: Healthy vertebrate cells display “self” MHC molecules. If infected by a pathogen, the cell will add a fragment of the pathogen’s antigen molecule to its own MHC.
  • Gene Pool: The collection of all the genes and the various alleles of those genes within a population.
  • Natural Selection: Will act on the alleles within the gene pool, maintaining alleles that provide an advantage and selecting out alleles that negatively impact survival and/or reproduction.
  • Maximum Number of Alleles: Even if there are more than two alleles in the gene pool, there are a maximum of two alleles in any single individual.
  • S-gene Example:
    • Gene with multiple alleles (over 50 known alleles of the S- gene in apples).
    • The alleles are numbered S1, S2, S3, etc
    • Cross-pollination is necessary to achieve sexual reproduction.
    • During pollination, if a pollen lands on the stigma of a flower with the same allele of the S-gene, the pollen is rejected, and fertilization will not occur.

D3.2.9 ABO Blood Groups as an Example of Multiple Alleles

  • ABO Gene: An example of a gene with multiple alleles that codes for an enzyme that alters the structure of a glycoprotein on the red blood cell membrane.
  • Alleles: The gene has three alleles, IA, IB and i.
    • Allele IA: Alters the glycoprotein by adding a molecule called acetylgalactosamine.
    • Allele IB: Alters the glycoprotein by adding a molecule called galactose.
    • Allele i: Adds nothing to the glycoprotein.
  • Individual Alleles: An individual person has a max of two alleles, one from their mom and one from their dad.
  • Combinations:
    • IAIA, IAIB, IAi, IBi, IBIB, ii
  • Dominance:
    • Alleles IA and IB each show complete dominance over i.
    • Alleles IA and IB are codominant
  • Phenotypes:
    • Type A: IAIA or IAi
    • Type B: IBIB or IBi
    • Type AB: IAIB
    • Type O: ii
  • Blood Type Cross Example: One parent has type AB blood and the other has type O blood. What are the expected blood type genotype and phenotype ratios in the offspring?
    • A, B
  • Blood Type Cross Example: One parent has type A blood and the other has type O blood. They have a child with type O blood. What is the genotype of the type A parent?
    • IAi

D3.2.10 Incomplete Dominance and Codominance

  • Incomplete Dominance: The heterozygous phenotype is an intermediate because one allele is not completely dominant over the other alleles of a gene.
    • Example: In the snapdragon (Antirrhinum majus) flower, color is influenced by a gene with two alleles, red (CR) and white (CW). If cross-pollination (D3.1.10) occurs between a homozygous parent with white flowers (CWCW) and a homozygous parent with red flowers (CRCR), the offspring will have pink flowers (CRCW).
  • Codominance: The heterozygote expresses the phenotype of two alleles equally. The phenotype is not a blending, rather equal and independent expression of two phenotypes.
    • Example: Roan coat color in many organisms is the result of having red hair (CR) interspersed with white hairs (CW). Both hair color alleles are expressed.
  • Codominance in ABO Antigens:
    • Red blood cells have glycoproteins on their their plasma membranes that distinguish ABO blood type. The glycoproteins are coded for by one gene with three alleles, IA, IB and i. Both the IA and IB alleles of the gene show dominance over i. However neither IA or IB is dominant over the other, they are codominant.

D3.2.11 Sex Determination in Humans and Inheritance of Genes on Sex Chromosomes

  • Sex Determination: Sperm cells determine the sex of offspring.
  • Genetic Diagram: The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes
  • Gametes:
  • Mother (XX)
  • Father (XY)
  • Offspring Ratio: 1:1
    • 50% chance of a boy (XY)
    • 50% chance of a girl (XX)

D3.2.12 Hemophilia as an Example of a Sex-Linked Genetic Disorder

  • X and Y Chromosomes:
    • The X chromosome is larger than the Y, and has its centromere more central than on the Y chromosome.
    • Fewer genes are coded for on the Y chromosome as a result.
    • The X carries around 16 x more genes than the Y chromosome.
    • Non-sex phenotypic traits, including certain blood clotting factors, are coded for on the X chromosome but not on the Y.
  • Y Chromosome:
    • The Y chromosome carries genes that code for male characteristics
      • One of these genes is the SRY gene which in involved in: Development of testes in male embryos and production of testosterone
  • Sex-Linked Genes: Sex-linked genes are those found on either the X or Y chromosome. Most sex-linked traits are due to genes located on the X chromosome because there are many more genes on the X chromosome than on the Y chromosome.
  • Sex Linked Gene Notation:
    • Male XA Y
    • Female XA Xa
  • Non-diseased homozygous female XA XA
  • Heterozygous female (“carrier”) XA Xa
  • Diseased female Xa Xa
  • Normal male XA Y
  • Diseased male Xa Y
  • Hemophilia:
    • XH: The normal (non-mutated) hemophilia alleles provide instructions for making proteins that work together in the blood clotting process.
    • Xh: The mutated hemophilia alleles code for altered or missing proteins that cannot participate effectively in the blood clotting process.
  • Haemophilia;
    • Woman who is a carrier for haemophilia is pregnant. The father does not have haemophilia. What are the odds their child will have haemophilia?
    • 1/4

D3.2.13 Pedigree Charts to Deduce Patterns of Inheritance of Genetic Disorders

  • Pedigrees: A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes of a particular gene from one generation to the next.
  • Deducing Patterns:
    • Does the trait skip generations?: Yes - the trait is recessive; No - the trait is dominant
    • Is the trait found much more often in males?: Yes - the trait is sex linked; No - the trait is autosomal

D3.2.15 Box-and-Whisker Plots

  • Qualitative: data that is descriptive using words and is open to interpretation
  • Quantitative:
    • Discrete: a finite value that can be counted
    • Continuous: an infinite number of possible values can be measured
  • Data Set: A collection of data. Data sets are often groupings of measurements of the same thing, for example the multiple trials of data collected to strengthen the reliability of data.
  • Histogram: represents the distribution of quantitative data in a single set
  • Bar Chart: is used to compare quantitative data in qualitatively different data sets
  • Line/Curve Plot: is used to compare quantitative data in quantitatively different data sets
  • Box and Whisker: is used to compare quantitative data in different data sets. The X axis is the comparative group and the Y axis is the median value for each data set. Error bars can be added to display the variation within a data set, using the interquartile range.
  • Box-and-whisker plots display six aspects of data:
    • Outliers
    • Minimum
    • First quartile
    • Median
    • Third quartile
    • Maximum
  • Quartiles: divide a data set ordered from smallest to largest into four equal parts. The median is the middle number of the ordered data set (the second quartile, Q2).
  • Outlier: An outlier is an observation that lies an abnormal distance from other values in a random sample from a population.
  • IQR and Outliers: If a data set containers an outlier and is skewed, then the data is best: described with the median as the central tendency of the data set described with the IQR as the measure of spread within the data set represented with a box-and-whisker plot.
    If a data set does not container an outlier and is not skewed, then the data is best: described with the mean as the central tendency of the data set described with the standard deviation as the measure of spread within the data set.