D3.2 - Inheritance

Question 1

Sexual Reproduction

Answer 1

• Requires two parents
• Each parent contributes a haploid gamete
  produced by meiosis
• Animals: sperm and eggs
• Plants: pollen and ova
• Gametes combine during fertilisation to create a diploid zygote (which develops into an embryo)
• Most plants and animals reproduce through sexual reproduction

Question 2

Sexual Reproduction

what happens before fertilisation

Answer 2

• Before fertilisation, the parent cells must first undergo meiosis to create gametes (D1.2)
• Gametes carry half of the genetic information (haploid - only one copy of each
  chromosome)
• This ensures the correct number of
  chromosomes in offspring when gametes
  combine (diploid - two copies of each
  chromosome)

Question 3

Dominant and reccessive

Answer 3

• Dominant: an allele whose expression overpowers the effect of a second
  form of the same gene - always expressed
• Recessive: an allele whose effects are concealed by the dominant allele
  in a pair - NOT always expressed

Question 4

Genotype and what ar ethey characteristed as

Answer 4

an individual’s allele combination for any given gene or trait

• Genotypes can be characterised as:
• Homozygous: a condition in which two alleles for a given gene are the
  same; can be homozygous dominant or homozygous recessive
• Heterozygous: a condition in which two alleles for a given gene are
  different

Question 5

Phenotype:

Answer 5

observable characteristics of an organism, including physical, behavioural and physiological traits

Question 6

Phenotypic Plasticity

Answer 6

• Phenotypic plasticity refers to the ability of an organism to change its physical traits or
  characteristics in response to environmental factors, without altering its genetic makeup
• Essentially, it’s the capacity of an organism to adapt its appearance or behavior to better suit itsenvironment

Question 7

Arctic Fox an dphenotypic plasticity

Answer 7

• As phenotypic plasticity is not due to changes in genotype, the changes in traits may be reversible during the lifetime of an individual
• Arctic foxes (B4.1) use phenotypic plasticity to adapt to living in the harsh and cold
  environments of the arctic tundra
• During the winter months, the arctic fox has a thick, white coat that helps it to blend in with the snow and ice
• In the summer, their coat turns a brown or
  gray color, which helps it to blend in with the rocks and vegetation of the tundra

Question 8

Flowering Plants to observe changes in the
phenotypes of offspring

Answer 8

• This allows researchers to make predictions on patterns of inheritance
• In plants, the male gamete is called pollen
  (found on the anther) and the female gamete is called ovum (found inside ovules)
• The combination of these gametes is called
  fertilisation
• Many plants, including pea plants, contain both male and female gametes
• This means they can undergo self-pollination (leading to self-fertilization)
• The gametes of different pea plants can also be combined using cross-pollination, creating genetic diversity

Question 9

Observing Inheritance with Flowering Plants

benefits

Answer 9

• There are many benefits to using pea plants for genetic crosses, including:
• Plants can produce seeds in short periods of time, so lots of data can be collected quickly
• Plants have easily distinguishable characteristics that can be followed (ie: seed colour, plant height, flower colour)

Question 10

• Purebred
• Hybrid
• P (parent) generation
• F1 generation
• F2 generation

Answer 10

• Purebred - organisms with identical alleles for specific traits
• Hybrid - organisms with a combination of alleles for specific traits
• P (parent) generation - the first set of individuals crossed, usually
  purebred
• F1 generation - offspring resulting from the cross of the P generation
• F2 generation - offspring resulting from the cross of the F1 generation

Question 11

Conducting Genetic Crosses

Answer 11

1. A stock of pure-breeding plants is established by self-pollination (P generation)

Plants are considered pure-breeding if they
always produce offspring with the same
phenotype as the parent (homozygous)

2. Two pure breeding plants with different
   characteristics are cross-pollinated to
   produce hybrid offspring (F1 generation)

If the parental plants are homozygous, all
offspring in the F1 generation will be
heterozygous

3. Members of the F1 generation can be
   crossed to produce the F2 generation with
   a variety of characteristics

Some of the physical characteristics in the
F2 generation will not have been present in
the F1 generation (ie: white colour)

Question 12

Genetics Nomenclature

Answer 12

• When describing dominant alleles, we use
  CAPITAL letters
• When describing recessive alleles, we use
  lowercase letters
• Ex: Brown eyes - B
  Blue eyes - b

Question 13

Dominant vs Recessive

Answer 13

• Many traits follow a typical pattern of inheritance where the dominant allele is expressed over the recessive one
• Phenotypically, homozygous dominant and heterozygous forms will be indistinguishable from one another – the recessive allele will only be seen in the phenotype when in a homozygous state
• This pattern of inheritance is called complete dominance

Question 14

PKU

Answer 14

• In humans, chromosome 12 contains a gene that codes for the enzyme phenylalanine hydroxylase (PAH)
• This enzyme converts the amino acid
  phenylalanine (Phe) to tyrosine (Tyr)
• This is an important process, as high levels of phenylalanine can build up and become toxic to the brain and nervous system (leading to intellectual disability and damage)
• Phenylketonuria (PKU) is an autosomal genetic condition caused by a mutation to the PAH gene
• The mutation prevents the enzyme PAH from forming
• This means that Phenylalanine concentrations build up in the blood (as it is not converted to tyrosine by the enzyme), which has many negative developmental consequences for the individual

Question 15

PKU as a Recessive Disorder

Answer 15

• PKU is a recessive disorder, meaning the individual must inherit two recessive alleles from their parents
• Use punnets squares to show how this condition could be passed on to offspring (note: there are 3 ways!)
• Scenario 1: both parents must be carriers of the allele, meaning they are heterozygous for the trait
• Scenario 2: one parent has PKU, the other parent is a carrier
• Scenario 3: both parents have PKU

Question 16

Incomplete Dominance

Answer 16

• Incomplete dominance occurs when neither allele is completely dominant
• The heterozygous phenotype is an
  intermediate between the two homozygous
  phenotypes
• An example of incomplete dominance can be seen in the flower colour of the four o’clock flower (Mirabilis jalapa)
• When the red and white coloured flowers
  cross-pollinate, a new phenotype of pink
  flowers is seen in the F1 offspring

Question 17

Co-Dominance

Answer 17

• Co-dominance occurs when pairs of alleles
  are both equally expressed in the phenotype of a heterozygous individual
• Both phenotypes are visible at the same time
• A common example of co-dominance is in
  the ABO blood group (we will look at this in
  a few slides)

Question 18

Incomplete & Codominant Conventions

Answer 18

• Use superscripts for the different incomplete or co-dominant alleles (recessive still lower case)
• Choose a common base letter (in this example C for “colour”)
• Choose different superscript letters to represent the alleles (in this case B for “black” and W for “white”)
• Ex: feather colour in chickens is co dominant, resulting in a speckled coat

Question 19

Multiple Alleles

in relation for SNPs

Answer 19

• Recall: SNPs change the DNA nucleotide sequence, leading to new variations of a gene (alleles) (D1.3)
• Since SNPs lead to phenotypic variation, most genes have more than just a single dominant and recessive variant
• This means that within a population gene pool, there is more than one allele for the same gene – this is called multiple alleles
• Even though there can be multiple alleles for the same gene in a population, an individual will only have two copies of the gene, inheriting one allele from each parent

Question 20

ABO Blood Group

Answer 20

• ABO blood type in humans is a common example of both multiple alleles and co-
  dominance
• Blood group can be determined by three
  different alleles: A, B and O
• A and B are co-dominant
• O is recessive

Question 21

Blood Group Glycoproteins

Answer 21

• Red blood cells contain glycoproteins
  (antigens) on their surface (B2.1)
• Antigens act like markers that allow the body to recognise the blood cells as self
• Depending on your blood type, your immune system will produce antibodies to react against other blood types
• Type A - has A glycoprotein, produces B antibodies
• Type B - has B glycoprotein, produces A antibodies
• Type AB - has A and B glycoproteins, produces neither of the
  antibodies
• Type O - has neither A nor B glycoproteins, produces both A and B
  antibodies

Question 22

Types of Chromosomes

Answer 22

• Humans have 23 sets of chromosomes
• 1-22 = autosomes
• 23 = heterosomes

Question 23

Autosomes

Answer 23

• Determines somatic traits
• Males and females have the same copy of
  autosomes
• Labelled with numbers (1 to 22)
• Pairs of autosomes are homologous
• Contain many genes, ranging from 200 to
  2000

Question 24

Sex Chromosomes

Answer 24

• Determines sex
• Different in males and females by their size
  and form
• Labelled with letters (X and Y)
• Female are homologous (XX);
  male are non-homologous (XY)
• X chromosomes has more than 300 genes;
  Y chromosome has only a few genes

Question 25

Sex-Linked Traits

Answer 25

* Sex linkage refers to a gene located on a sex chromosome (X or Y) * The Y chromosome is much shorter than the X chromosome and contains fewer genes * The X chromosome contains many genes not present on the Y chromosome

Question 26

The Y Chromosome

Answer 26

* The Y chromosome contains the genes for developing male sex characteristics * In the absence of a Y chromosome, female sex organs will develop * The father is therefore responsible for determining the sex of offspring

Question 27

Sex Determination

Answer 27

* Embryonic development is the same in all embryos in the initial stages * Embryonic gonads either become ovaries or testes depending on the presence of one gene found on the Y chromosome - SRY gene

Question 28

Sex-Linked Inheritance

Answer 28

* Sex-linked inheritance patterns differ from autosomal patterns since the chromosomes aren’t paired in males (XY) * This leads to the expression of sex-linked traits being predominantly associated with a particular sex

Question 29

Sex-Linked Possibilities

Answer 29

* Male: affected, not affected * Female: affected, not affected, carrier

Question 30

Sex-Linked Conventions

Answer 30

* When assigning alleles for X-linked traits, the convention is to write the allele as a superscript to the sex chromosome (X) * Ex: dominant trait XAXA XAXa XAY

Question 31

Sex-Linked Recessive

Answer 31

* X-linked recessive disorders are much more common in males than in females * Ex: haemophilia, red/green colour blindness * A female must have two of the abnormal alleles in order to express the disease * However, if a male has one abnormal allele, they will express the disease

Question 32

Haemophilia

Answer 32

* The formation of a blood clot is controlled by a gene located on the X chromosome * Haemophilia is a recessive genetic condition in which this gene is affected, meaning blood-clotting proteins are not produced * An individual with haemophilia is at risk of bleeding excessively, even from relatively small traumas * Unaffected (normal blood clotting): XH * Affected (haemophilia): Xh

Question 33

Pedigrees

Answer 33

* Tool used to study inheritance patterns of specific traits in a family * Similar to a flow chart depicting the different generations within a family * Allows scientists to determine if a trait is dominant or recessive * Often used to follow the inheritance of a genetic disorder

Question 34

Steps to Interpret Pedigrees

Answer 34

1. Determine if the pedigree chart shows an autosomal or X-linked disease * If most of the males in the pedigree are affected, then the disorder is most likely X- linked * If it is 50/50 between affected men and women, the disorder is most likely autosomal 2. Determine whether the disorder is dominant or recessive * If the disorder is dominant, one of the parents must have the disorder * If the disorder is recessive, neither parent has to have the disorder (they can be heterozygous)

Question 35

Types of Variation

Answer 35

Discrete Variation Continuous Variation Definition: Clearly defined categories of a characteristic that can be observed A range of phenotypes from one extreme to another Cause: Characteristic controlled by a single gene (monogenic traits) Characteristics controlled by many genes (polygenic traits) and often influenced by environment Phenotype: Individuals express one of a number of distinct phenotypes An individual’s phenotype lies somewhere along a continuous spectrum Example: blood type (ABO) height

Question 36

Continuous Variation what is an exmaple

Answer 36

* Melanin pigment causes brown colouration in skin * There are over 150 genes that impact skin colour, either directly or indirectly! * Dominant alleles promote melanin * Recessive alleles do not promote melanin * The more alleles someone has promoting melanin pigment, the darker their skin

Question 37

Assumptions of Polygenic Model

Answer 37

* Each contributing gene has small and relatively equal effects * The effects of each allele are additive * The genes behave as if they are co-dominant

Question 38

Box and Whisker Plots

Answer 38

* Box and whisker plots are used to show the variation of the data (spread) for a continuous variable * The minimum and maximum values are at the ends of each whisker * The median value for the data is within the box * The lower quartile (Q1) and upper quartile (Q2) values are at either end of the box * The length of the box represents the interquartile range Outliers * An outlier is a data point that differs significantly from other data points * A data point is categorized as an outlier if it is more than 1.5 × IQR (interquartile range) above the third quartile or below the first quartile * Outliers are removed from the data set, and a box and whisker plot is constructed without the outlier * The outliers are indicated on the graph by *

Question 39

Law of Independent Assortment

Answer 39

* The Law of Independent Assortment states that alleles of two (or more) different genes are sorted into gametes independently of one another * The separation of homologous chromosomes during meiosis I separates the two alleles * Therefore, if genes are on different chromosomes, their alleles will be independently assorted * These genes are said to be unlinked

Question 40

Dihybrid Crosses

Answer 40

* Determines genotypic and phenotypic combinations of offspring for two particular genes that are unlinked * Since each gene has two alleles, there can be up to four different gamete combinations Example: * S and Y represent two different genes * The parents are heterozygous for each trait (Ss and Yy)! The easiest way to work out potential gamete combinations in a dihybrid cross is to use the FOIL method:

Question 41

Steps to Complete a Dihybrid Cross

Answer 41

Step 1: Choose different letters to represent each gene Step 2: Assign characters to represent the alleles * CAPITAL - dominant allele * lower case - recessive allele Step 3: Write down the genotype and phenotype of the parents * Always pair alleles from the same gene and always write capitals first (e.g. AaBb, not ABab) Step 4: Determine potential gamete combinations for both parents * Use the FOIL method Step 5: Create a Punnett square to work out potential genotypes of offspring Step 6: Determine the phenotypic ratios of potential offspring

Question 42

Linked Genes

Answer 42

* Linked genes are those found on the same chromosome and therefore do not assort independently * They do not follow normal inheritance patterns and are often inherited together * Linked genes may become separated due to recombination (crossing over), but this is not guaranteed

Question 43

Conventions for Linked Genes

Answer 43

* Linked genes are represented as lined pairs: A ll a b II B * The two lines represent homologous chromosomes in an individual * Letters “A” and “a” represent two different alleles for the same gene * Letters “A” and “B” represent two different genes found on the same chromosome

Question 44

Recombinants

Answer 44

* Recombinants indicate an organism with a different combination of alleles than either of its parents * Recombinants can occur in both unlinked and linked genes

Question 45

Recombinants: linked Genes

Answer 45

* In genes that are linked, recombinants will arise as a result of crossing over during meiosis (prophase I) * If linked genes become separated by a chiasma, there will be an exchange of alleles between the non-sister chromatids * In linked genes, the formation of recombinant alleles is dependent on the distance between the genes * Crossing over (forming recombinants) is more likely to happen if the genes are further apart on the chromosome

Question 46

Identifying Linked Genes

Answer 46

* Since linked genes do not move independently of each other, they do not follow traditional patterns of inheritance, and therefore do not show expected phenotypic ratios * The easiest way to test for linkage between two genes is to use a test cross and observe the ratios * A heterozygous parent is crossed with a homozygous recessive parent * If the genes are unlinked, we can expect typical phenotypic ratio from this cross (1:1:1:1) * However, if the genes are linked, the recombinant phenotypes will occur less frequently as crossing over is not guaranteed to happen

Question 47

Chi-Square Test formula and how to calculate

Answer 47

* A Chi-squared test is a method of determining whether the difference between observed and expected frequencies is statistically significant * It can be used to determine if the genes are linked or unlinked * The formula is: x² = ∑(O-E)² ÷ E (x²): The calculated value. (∑): Sum the result of the calculation for all categories. (O): The actual frequency collected in the data. (E): The theoretical frequency. Chi-squared test 4 steps: 1. Identify hypotheses (null and alternative) 2. Construct a table of frequencies (observed and expected) and apply the chi-squared formula 3. Determine critical value 4. Test for significance by comparing the chi-square value to a critical value