Genetics

Learning Target Objectives: (I can …)

• Analyze pedigrees, determining who has, carries, or is free of a trait.• Create Punnett squares for monohybrid and dihybrid crosses• Predict the genotype, phenotype and the ratios of each in the offspring of a

homozygous, heterozygous, etc. cross.• Compare and contrast inheritance for sex-linked traits, codominance,

incomplete dominance, simple dominant and recessive traits, and multiple allele situations.

• Explain the significance of Gregor Mendel’s work and the use of a testcross.• Apply Mendel’s 3 principles• List common genetic disorders or diseases

Vocabulary:

Gregor Mendel * heredity * pure * strain * self-pollination * pollination * cross-pollination * P1 * F1 * F2 * gene * allele * dominant * recessive * genotype * heterozygous * homozygous * phenotype * multiple alleles * monohybrid cross * Punnett square * probability * gametes * testcross * codominance * incomplete dominance * dihybrid cross * germ cells * somatic cells * mutation * deletion * inversion * translocation * nondisjunction * monosomy * trisomy * point mutation * frameshift mutation * mutagens * autosomes * sex chromosomes * sex-linked trait * linkage group * population sampling * pedigree * carriers * codominant

Genetics

Gregor Mendel – statistically determined the probability for the appearance of certain traits. (Ex: What are the chances of a yellow bean being produced when a green bean and a yellow bean are crossed?)

Heredity – transmission of traits from parents to offspring

Pure – offspring will always have that trait (Ex: always have red flowers)

Strain – All plants in family lineage are “pure” for that specific trait

Self-pollination - fertilization occurs between flowers on the same plant (Or it may be between the male/female parts of the same flower)

Pollination – pollen transfer from anther to stigma

P1 – (parental generation) a strain used for pollination/genetic tests

P1 x P1 Cross-pollination (green pod) (yellow pod) Must be 2 different plants since

these are pure for their trait

F1 and 1 is green and 1 is yellow

F1 (first filial generation) – offspring of the P1 generation (a cross of 2 different pure strains)

F2 (second filial generation) - is produced by self-pollinating the F1 generation

F1 x F1 Self-pollination

F2

Gene – segment of a chromosome that has nucleotide codes for a certain trait

Allele – the contrasting, or partner gene, for a given trait. There is 1 allelle on each homologous chromosome for a total of 2 alleles per trait. (Assuming only 1 set of alleles controls that trait.)

Alleles are labeled to indicate dominance. For example, since green pods are dominant over yellow, the alleles for pod color are labeled:

G (to show the dominant green allele is present) A capital letter is always used to show the dominant allele.

or

g (to show the recessive yellow allele) A lower case letter (of the same letter of the alphabet) is always used for the recessive allele.

A pure green plant would have both alleles for green. It is: GGA pure yellow plant would have both alleles for yellow. It is: gg

Mendel found the F1 generation often only expressed (showed) one trait. Ex: crossing peas of pure green pods to peas with pure yellow pods produced peas with all green pod offspring. So he came up with “The Principal of Dominance and Recessiveness.”

Dominant allele – a gene that masks its partner gene on the homologous chromosome. Ex: If “Dad” gives a smooth pea gene (allele) and “Mom” gives a wrinkled pea gene (allele)but the offspring are all smooth, we know smooth is dominant because it is expressed.

Recessive allele – these alleles are masked by dominant alleles. They are NOT expressed traits unless BOTH alleles are for the recessive trait. So, if only 1 of the 2 alleles is dominant, the offspring only show the dominant trait.

Mendel’s 3 Principles:

1) Principle of Dominance and Recessiveness: 1 factor (dominant) may mask another (recessive) preventing it from being seen.

2) Principle of Segregation: “Factors” (genes) separate during egg or sperm production so offspring will have a total of 2, not 4, factors for that trait.

3) Principle of Independent Assortment: Factors (genes) for different traits are not connected. In other words, a plant dominant for green pods could show the recessive trait for height.

Genotype – the actual genetic makeup of an organism. This can be shown using trait alleles. Exs:

GG - means both of the plant’s alleles are for the dominant green pod

Gg - shows that the pea pod is green but the plant has a recessive gene for yellow pod color.

gg - shows the pod will be yellow because both alleles code for the recessive yellow color.

Homozygous traits - BOTH alleles for the trait are the same. Ex: GG or ggHeterozygous traits - means the alleles for that trait are different Ex: Gg

Phenotype – the appearance of an organism (The expressed trait) Ex: GG and Gg are 2 different genotypes but since both code for green pods, they have the same phenotype.--- --- --- --- --- --- --- --- ---Multiple alleles – 3 or more alleles (in the general population) control a trait. Blood can be A, B, AB, or O type blood. Alleles for blood include IA, IB, and i. But still only 2 alleles can be found in any ONE individual. The other alleles, however, are found in the population at large. (Remember we get only 1 allele from each parent for a total of 2.)

Remember: A difference in phenotype (appearance) indicates a difference in genotype BUT this is not necessarily true in reverse.Ex: Gg and gg different phenotype and different genotype GG and Gg different genotype but SAME phenotypeHowever, phenotype is determined by genotype (Ex: You can’t get yellow from Gg or GG, you have to have a gg genotype)

Monohybrid cross – breeding together organisms that differ only in one trait (That is, only one trait is being considered).

Punnett square - shows the possible genotypes that might result from a cross.

Ex: Punnett square for a monohybrid cross looking at pod color

G G Shows 1 parent’s genotypeg Gg Ggg Gg Gg Boxes show possible genotypes of offspring

Shows the other parent’s genotype

In this example, one parent is pure for green (homozygous dominant) and one parent is pure for yellow (homozygous recessive). The offspring are all green heterozygotes (carry the yellow gene)

Punnett squares can be used to show probability (how likely the offspring are to inherit a certain genotype or how likely they are to exhibit a certain phenotype). It shows possible genotype combinations in the offspring.

G g Mom’s allelesG GG Gg Shows the only 4 unions of gametes that

Dad’s alleles g Gg gg can occur.

The genotypes show a 2 out of 4 chance (likelihood) of carrying the recessive “g” allele. (50% chance)The phenotype = a 1 out of 4 chance (25%) of being yellow, 3 out of 4 chance (75%) of being green.

Gametes – the sex cells (egg and sperm) Because we get only one set of chromosomes from each parent (meiosis), if they have an allele for a green pod on one chromosome and an allele for yellow on the other homologue (chromosome partner), we don’t know which allele the offspring might get (50% chance of getting either one from that parent).

Testcross – breeding a phenotypically dominant individual to a homozygous recessive individual to determine the genotype of the phenotypically dominant individual. (We know any individual showing the recessive trait is homozygous recessive for that trait. However, a phenotypically dominant individual could be homozygous dominant or heterozygous.)

Exs: G ? G G ? gg Gg Gg g Gg ggg Gg Gg g Gg gg

Phenotypically, all offspring Phenotypically, some willare green If parent is have yellow pods (50%),homozygous dominant (GG). If parent is heterozygous

(Gg).

Single gene cross possibilities:

Homozygous dominant x homozygous recessive:

G G Offspring are 100% green phenotype.g Gg Gg Genotypically, they are 100% Gg (all g Gg Gg “carry” the yellow gene – heterozygous)

Homozygous x heterozygous crosses:

G G g gG GG GG G Gg Ggg Gg Gg g gg gg

Phenotype: 100% green Phenotype: 50% green, 50% yellowGenotype: 50% homozygous dom.(green) Genotype: 50% homo.recessive (yellow)50% heterozygous (Gg) 50% heterozygous (Gg)

Heterozygous x heterozygous cross: G g Phenotype: 75% green & 25% yellow (3:1 ratio)G GG Gg Genotype: 25% homozygous dominant, 50% heterozygous,g Gg gg 25% homozygous recessive (1:2:1 ratio)

Codominance and Incomplete dominanceBOTH alleles influence the trait. With codominance, if a red flower allele were united with a white flower allele, the offspring would have red and white striped flowers or some other combination of red and white.With incomplete dominance, if a red flower allele were united with a white flower allele, the offspring would have pink flowers.

Usually, with codominant traits, one uses the capital letter and the other uses the same capital letter but with a prime sign.Ex: Red = R and White = R’ RR’ = red and white (or pink in incomplete

dominance)

Dihybrid cross - looks at 2 pair of alleles and the chance of getting both dominant traits, both recessive traits, etc. This yields 16 possible gene combinations in a heterozygous x heterozygous cross for both traits. From these 16 genotypes, there are only 4 phenotypes . There will be:

9 phenotypically dominant in both traits3 phenotypically dominant in one trait3 phenotypically dominant in the other trait1 recessive in both traits

***That’s a 9:3:3:1 ratio.Ex: AaBb x AaBb parents can produce eggs or sperm with

these combinations: AB, aB, Ab, ab which we can plug into a Punnett square.

AB aB Ab abAB AABB AaBB AABb AaBbaB AaBB aaBB AaBb aaBbAb AABb AaBb AAbb Aabbab AaBb aaBb Aabb aabb

Germ cells = sex cells (eggs or sperm --> gametes) (1N) (formed via meiosis)Somatic cells = body cells (2N) (divide by mitosis)

Mutation – a change in DNA (usually a change in base sequencing which can change which proteins are or are NOT made). When germ cells are affected, the offspring may show a deformity.

Chromosome mutations:

Deletion – a piece of chromosome breaks off so all of that information is lostInversion – a piece of chromosome breaks off, flips around and reattaches upside down in place.Translocation – a chromosome piece breaks off and reattaches to a different, nonhomologous chromosome.Nondisjunction – during meiosis, either homologous chromosomes or sister chromatids fail to separate (anaphase I or anaphase II) so 1 daughter cell gets an extra chromosome and 1 daughter cell is missing a chromosome. (This may also happen during mitosis but usually is not as serious.)

Abnormal Anaphase I: Abnormal Anaphase II:

X X X

nothing XX l l nothingAnaphase ll: gametes

nothing nothing l l l l should only be 1 chromatid in these

gametes

Nondisjunction causes:Monosomy – missing 1 chromosome out of the entire set (Ex: 45 instead of 46)

Trisomy – has 1 extra chromosome Ex: 47 instead of 46 chromosomesDown’s Syndrome – retardation & swollen face due to trisomy 21.Klinefelter’s syndrome – extra “X” chromosome (trisomy 23), low fertility, less male chest hair, etc., may be learning disabled. (XXY) This is a male condition. Turner’s Syndrome – (monosomy 23) female; will be underdeveloped and sterile (X “0”)

Homologous chromosomes fail to separate

Meiosis I:Nondisjunction

Meiosis II

Section 14-2

Nondisjunction

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Gene mutations:

Point mutations – change in 1 base (A, T, C, G) on a gene. It may be a substitution of the wrong base, an addition of a base, or the loss of a base. This can result in the wrong amino acid(s)/protein being formed.

Frameshift mutation – a point mutation where a base is added or deleted. This results in all of the following bases read out of order.(*Don’t need to draw this.)Ex: ACT TCA GCT CCG 3 bases are read together to code for an a. acid

AGC TTC AGC TCC G… Inserting 1 base changes EVERY codon for every amino acid from that point on.

Mutagens – anything that damages DNA (Ex: sunlight, radiation, cigarettes)

Sex Chromosomes = X and Y (chromosome #23) In humans, females have XX and males have an XY combination. Since only males carry the Y chromosome, they control the child’s sex.

X X Mom’s genes Dad’s X XX XX Daughters Genes Y XY XY Sons

Autosomes – all chromosomes NOT involved in sex determination (#1 – 22)

Sex-linked trait : genes for a trait may be carried on only one type of sex chromosome (Usually the X chromosome) and therefore may only appear in one sex (males) most of the time (Ex: color blindness & hemophilia).

Moms act as carriers (or have subclinical symptoms).Linkage group – group of genes found on one chromosome. These USUALLY travel together & are inherited as a group. (Except sometimes with crossing-over). But, the closer 2 genes are on a chromosome, the less likely they’ll be affected by crossing over.

Studying human traits may involve population sampling. Here a small cross-section of the population is used to represent the entire population. (Ex: 100 people are randomly selected and 40% have hairy knuckles, then it might be assumed that 40% of the entire population has hairy knuckles.)

Pedigree – record of several generations showing how traits were inherited. By analyzing a pedigree, we can sometimes find carriers (Someone who has 1 recessive gene but does not show it in the phenotype – heterozygous).

For a recessive trait, we use these symbols: Males: Females:

Homozygous dominant(non-carrier)

Heterozygous (carrier)

Homozygous recessive(displays recessive trait)

A circle represents a female.

A square represents a male.A horizontal line

connecting a male and female represents a marriage.

A vertical line and a bracket connect the parents to their children.A half-shaded circle

or square indicates that a person is a carrier of the trait.

A completely shaded circle or square indicates that a person expresses the trait.

A circle or square that is not shaded indicates that a person neither expresses the trait nor is a carrier of the trait.

Section 14-1

Figure 14-3 A Pedigree

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Single Allele Traits – only 1 allele controls the trait.

Ex: Sickle Cell Anemia – A = normal gene, A’ = sickle shape gene These are codominant (both are expressed if present):AA = all normal red blood cellsAA’ = ½ normal and ½ sickle cellsA’A’ = Sickle Cell Disease (all cells are sickle shaped)

AA’ (similar to carriers) – have resistance to malaria and mild or no sickle cell disease symptoms.

Ex: Huntington’s Disease – here a dominant gene exists for the disease but doesn’t appear until adulthood.

Polygenic Traits:Traits controlled by more than 1 set of genes. (Ex: skin and eye color – how dark the color will be.)

Multiple Allele Traits: One set of genes controls the trait but there are 3 or more alleles possible (in the population’s gene pool). Ex: Blood types: IA, IB, iO

Sex-influenced Traits:The trait, in heterozygotes, is only expressed in males, for example.

Ex: Baldness:BB - both men and women go baldBB’ - Only men go baldB’B’ - neither goes bald

(Hormones affect the trait.)

Ways to Detect Genetic Disorders:

Karyotype – a picture of each chromosome and its homologue. These are looked at for abnormalities.

Amniocentesis – fluid is removed from the embryonic sac. The chromosomes from cells are then karyotyped.