Topic 1: human genetics Flashcards
1
Q
Define genetics
A
Scientific study of heredity + variation
2
Q
Define heredity
A
Transmission of traits from 1 generation > next
3
Q
Define variation
A
Differences in appearances in offspring from parents/siblings
4
Q
Describe Mendel + his ideology
A
- Austrian monk = discovered basic heredity principles via pea breeding AKA father of genetics
2 LAWS OF INHERITANCE:
1) Law of segregation
2) Law of independent assortment
5
Q
Describe the 5 advantages of using pea plants for genetic study
A
1) Characters = distinct heritable features e.g. flower color + height + flower position + seed color + seed shape + pod color + pod shape
2) Traits = character variants e.g. purple/white flowers
3) Mating of plants can be controlled
4) Each plant has stamen + carpel
5) Cross-pollination = fertilization between different plants via dusting pollen from 1 onto another
6
Q
Describe the experimental approach of genetic crossing
A
- Remove stamen from purple flower
- Transfer sperm pollen from white flower to egg carpel of purple flower
- Pollinated carpel matures into pod
- Plant seeds from pod
- Examine offspring = all purple
7
Q
What are the different generations in genetic crossing?
A
- P generation = parental
- F1 generation = 1st offspring
- F2 generation = 2nd offspring via F1 mating
8
Q
Explain the law of segregation
A
- Hybridization: Mendel mated 2 contrasting true-breeding varieties
- True-breeding varieties: homozygotes = plants produce same variant when self-pollinating
-Result of mating white + purple = F1 all purple - Result of mating F1 hybrids = heterozygotes = 3:1 > purple:white in F2
- Therefore purple = dominant trait + white = recessive trait
9
Q
Define Mendelian inheritance
A
- 3:1 inheritance pattern in 6 other pea plant characteristics in F2 gen
- Mendel didn’t know about genes = called them heritable factors
10
Q
Define homozygous
A
- 2 identical alleles of a character = true breeding
11
Q
Define heterozygous
A
- 2 different alleles = hybrids
12
Q
Describe concept 1 of Medel’s model
A
- Alternative versions of genes account for variations in inherited characters
- Alternative versions = alleles
- Each gene is on a specific locus on specific chromosome
13
Q
Describe concept 2 of Mendel’s model
A
- For each character organism inherits 2 alleles = each parent
- Deduction made without the knowledge of genes on chromosomes
- Homozygotes = 2 alleles at locus identical
- Heterozygotes = 2 alleles at locus different
14
Q
Describe concept 3 of Mendel’s model
A
- If 2 alleles at locus differ = then the dominant allele determines organisms phenotype + other recessive allele has no effect on appearance
- Purple flower = dominant
- White flower = recessive
15
Q
Describe concept 4 of Mendel’s model
A
- AKA law of segregation
- 2 alleles for a gene = separate during gamete formation = meiosis = end up in different gametes
- Segregation of alleles = separation of homologous chromosomes
- Gametes = 1/2 homologous chromosomes from each pair in a somatic cell
16
Q
Define punnet square
A
- Diagram for predicting results of genetic cross of known genotype
- Capital = dominant
- Lowercase = recessive
17
Q
Define testcross
A
- If you cannot tell the dominant is homozygous/heterozygous = do testcross
- Breed unknown with homozygous recessive = if display recessive phenotype = unknown is heterozygous
18
Q
What is the ratio if the genes are located on the same chromosome?
A
3:1
19
Q
What is the ratio if the genes are located on different chromosomes
A
9:3:3:1
20
Q
Describe law of segregation
A
- Inheritance of 1 character
- F1 offspring = monohybrids
21
Q
Describe law of independant assortment
A
- Inheritance of 2 characters at the same time
- F1 offspring = dihybrids
- Non-homologous chromosomes
22
Q
What is the function of a dihybrid cross?
A
- To see if 2 characteristics are transmitted to offspring together of independently
- If genes on same chromosomes = inherited together = law of independent assortment doesn’t apply
- If genes on different chromosomes = inherited independently = law applies
23
Q
Describe complete dominance
A
- Phenotypes off heterozygote + dominant homozygote = identical
- Expression of dominant allele
24
Q
Describe incomplete dominance
A
- Phenotype of heterozygotes = inbetween phenotype of 2 parental phenotypes
- Intermediate phenotype
- E.g brown + white fur = beige
- E.g. red +white flowers = pink
25
Describe codominance
- 2 dominant alleles affect phenotype in separate ways
- Both alleles expressed
- E.g. blood groups
26
What is the gene ratio for incomplete dominance?
1:2:1
27
Explain the blood groups
- A/B = codominant alleles = equally expressed
- O = recessive
- Determined by carbohydrates on RBC
- 4 phenotypes = A/B/AB/O
- More than 2 alleles = has 3 alleles for enzyme I that attaches carbohydrates = IA/IB/i
- A genotypes = IAIA or IAi
-B genotypes = IBIB or IBi
- AB genotype = IAIB
- O genotype = ii
28
Define pleiotropy
- Property of genes to have muliple phenotypic effects
- E.g. pleiotropic alleles responsible for many symptoms of CF + sickle cell
29
Define epistasis
- Phenomenon = effect of 1 gene modifies by other genes
30
Define polygenic inheritance
- 2+ genes control expression of single phenotype
31
Describe epistasis
- Gene at 1 locus alters phenotypic expression of gene at other locus
- Result = 2 different gene sets affect same phenotype
- E.g mice coat color depends on 2 genes = 1 gene determines pigment color + 1 determines if pigment will be deposited
- B = black / b = brown
- C = expression / c = inhibits
- cc = white
32
What is the gene ratio of epistasis
- B:b:cc
- 9:3:4
33
Explain epistasis in albinism
- Albinism = congenital autosomal recessive disorder = absensce of pigment
- Due to absence/defect of tyrosinase = produces melanin
34
Describe polygenic inheritance
- Additive effect
- Quantitative characters = charcters that vary in population = height + skin color
35
How many genes inherited for skin pigmentation?
- Atleast 3 genes = 6 alleles
36
Describe the phenotype
- Physical appearance
- Internal anatomy
- Physiology
- Behaviour
37
Why are humans not good subjects for genetic research?
- Generation time is too long
- Parents produce few offspring
- Breeding experiments are unacceptable
38
Define karyotypes
- Ordered display of pairs of chromosomes
- 22 pairs autosomes + 1 pair of sex chromosomes
39
Define homologous chromosomes
- Have same length + shape
- Carry gene controlling same inherited characters
- Don't necessarily carry same alleles
40
How many chromosomes in a somatic cells?
- 44 autosomes
- 2 sex chromosomes
- Total = 46 chromosomes
41
Define autosome
- Chromosomes don't determine the sex
42
Define pedigree
- Family tree describe interrelationships of parents + children across generation
- Inheritance patterns
- Prediction of future offspring
43
What are the types of inherited human disorder?
1) Autosomal dominant disorder
2) Autosomal recessive disorder
3) Sex-chromosome linked disorder
44
Describe autosomal recessive disorder
- Mutation in allele located in 22 pairs of autosomes = a = recessive
- Most common genetic disorders
- Parent may be heterozygous healthy carrier
- 2 heterozygous carriers = 25% probability to produce diseased offspring
- E.g. CF + SCA + thalassaemia + albinism
45
Describe sickle cell
- Caused by substitution of single amino acid Glu>Val in haemoglobin
- Survival advantage of being heterozygous = less susceptible to malaria
46
Describe thalassaemia
- Unaffected carrier parents = 25% chance diseased offspring
- Mutation in haemoglobin gene α/β chains = α/β thalassaemia
47
Describe cystic fibrosis
- Defective/absent chloride transport channels in plasma membranes
48
Describe autosomal dominant disorders
- Affected individuals don't survive to adulthood to produce offspring
- If survive = disorder present in phenotype = against natural selection
- Exception = disorders that express symptoms late in life = e.g. Huntington
- E.g. Huntington + achondroplasia
49
Describe Huntington
- Neurodegenerative disorder
- No phenotypic effect until 30-40
- Late onset affects inheritance = allows mating
- deterioration of NS = irreversible + fatal
50
Describe achondroplasia
- Form of dwarfism caused by rare dominant allele
51
Describe sex chromosome linked disorders
- Y has few genes = few Y-linked disorders
- Mostly X-linked dominant or recessive
- X-linked recessive more common than dominant
- X-linked recessive more common in men than women
52
Describe X-linked recessive
- More in men = absence of normal allele on Y chromosome = XA dominant
- If mother is carrier = 50% sons affected
- Mutant allele = Xa
- E.g. hemophilia + colorblindness + Duchenne muscular dystrophy
53
Describe X-linked dominant
- Rare + fatal
- Mutant allele = XA
- Females more affected than males
- Affected dad = 100% daughters 0% sons
- Affected mom = 50% children
- E.g. vitamin D resistant + rickets + Alport syndrome
54
Explain genetic counseling
- Using family pedigrees = help couple determine odds children will have genetic disorder
- Based on Mendelian genetics + probability rules
- Genetic testing = identify carrier e.g. 16% Cypriot carriers for thalassaemia
55
Describe prenatal testing
1) Amniocentesis = amniotic fluid removed + tested
2) Chorionic villus sampling = CVS = sample of placenta removed
3) Non-invasive prenatal diagnosis = NIPD = sample of mother's blood + fetal cells removed
- All followed by karyotyping = genetic testing
56
Describe newborn screening
- Detected at birth
- E.g. phenyketonuria screening
- If detected early = treat with diet low in phenylalanine
