Week 1

Question 1

Rhesus factor: pattern of inheritance - ?

Answer 1

autosomal dominant

Rh+ = D allele (dominant) rh- = d allele (recessive)

Question 2

Genetics -

Answer 2

scientific study of heredity and variation

Question 3

Heredity -

Answer 3

transmission of traits from one generation to the next (genes are acquired through chromosome inheritance)

Question 4

Variation

Answer 4

the differences in appearance that offspring show from parents and siblings

Question 5

Charcter & Traits:

Answer 5

Characters - distinct heritable features (such as flower colour)

Traits - character variants (such as purple or white flowers)

Question 6

Homozygous -

Answer 6

organism with 2 identical alleles for a character is homozygous for the gene controlling that character (e.g. AA or aa; true-breeding)

Question 7

Heterozygous

Answer 7

organism that has 2 different alleles for a gene is heterozygous for the gene controlling that character (e.g. Aa; not true-breeding, hybrids)

Question 8

Phenotype -

Answer 8

physical appearance (characteristics) of an organism

Question 9

Genotype -

Answer 9

genetic make up of an organism

Question 10

Alleles -

Answer 10

different versions of the same gene found on the same position on homologous chromosomes and responsible for encoding the same characteristic

Question 11

Dominant alleles -

Answer 11

the ones expressed in the phenotype (represented using capital letter)

Question 12

Recessive alleles -

Answer 12

not expressed in the phenotype when combined with a dominant allele, only expressed when combined with a recessive allele (represented using lower case letter)

Question 13

Mendel’s Model: 4 related concepts to explain the 3:1 inheritance pattern he observed in F2 offspring generation

Answer 13

1. alternative versions of genes account for variations in inherited characters (alternative versions of genes account for variations in inherited characters); each gene resides at a specific locus (position) on a specific chromosome. Mutation is the source of diff alleles.
2. for each character an organism inherits 2
   alleles, 1 from each parent
3. if the 2 alleles at a locus differ (in heterozygotes), then one (the dominant allele) determines the organism’s appearance (phenotype), and the other (the recessive allele) has no noticeable effect on appearance.
4. The law of segregation: - The 2 alleles for a gene (heritable character) separate (segregate) during gamete formation (meiosis) and end up in different gametes. Segregation of alleles corresponds to the separation of homologous chromosomes to different gametes during meiosis. Gametes (egg or sperm) have only 1 of the 2 homologous chromosomes from each pair that are present in the somatic cells of an organism => only 1 of the 2 alleles

Question 14

testcross -

Answer 14

breeding the unknown genotype individual with a homozygous recessive individual

Question 15

Law of segregation is based on

Answer 15

Monohybrid cross: F1 Aa x Aa = F2 is segregated phenotypically 3:1 and genotypically 1:2:1

Question 16

Law of independent assortment is based on

Answer 16

Dihybrid cross: F1 AaBb x AaBb = depending on whether characters are transmitted as a package or independently - the law either appies or not

Question 17

Dihybrid cross

Answer 17

Crossing 2 true-breeding parents (homozygotes) differing in 2 characters produces dihybrids in the F1 generation, heterozygous for both characters. A dihybrid cross (cross between F1 dihybrids) can determine whether 2 characters are transmitted to offspring as a package (together) or independently

• If 2 genes are located on the same chromosome => most likely they will be inherited together => law of independent assortment does not apply
• If 2 genes are located on different chromosomes => they will be inherited independently => law of independent assortment applies

Question 18

number of possible combinations in gametes:

Answer 18

2ⁿ, where n = haploid species chromosome number

Question 19

The Law of Independent Assortment:

Answer 19

each pair of alleles segregates independently of each other pair of alleles during gamete formation; it applies only to genes on non-homologous (different) chromosomes

Genes located near each other on the same chromosome tend to be inherited together (dependent assortment of linked genes)

Question 20

Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:

Answer 20

– When alleles are not completely dominant or recessive
– When a gene has more than 2 alleles
– When a gene produces multiple phenotypes

Question 21

Degrees of Dominance

Answer 21

• Complete dominance: occurs when phenotypes of the heterozygote and dominant homozygote are identical (expression of dominant allele phenotype)
• Incomplete dominance: when the phenotype of heterozygotes (F1 hybrids) is somewhere between the phenotypes of the 2 parental phenotypes (varieties) (neither allele is expressed; intermediate phenotype)
  ex: white carnation x red carnation => pink carnation, palomino horse (white + black => beige), skin colour in humans
• Codominance: 2 dominant alleles affect the phenotype in separate distinguishable ways (both alleles are co-expressed)
  ex: blood groups A and B

Question 22

Frequency of Dominant Alleles

Answer 22

Dominant alleles are not necessarily more common in populations than recessive alleles

Question 23

Pleiotropy -

Answer 23

property of most genes to have multiple phenotypic effects

ex: pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

Question 24

Epistasis -

Answer 24

phenomenon where the effects of 1 gene are modified by 1 or several other genes;
when a gene at one locus alters the phenotypic expression of a gene at a second locus => 2 different gene sets affect the same phenotypic characteristic

ex: in mice and many other mammals coat color depends on two gene: one gene determines the pigment color (with alleles B for black and b for brown), the other gene (with alleles C for colour and c for no colour) determines whether the pigment will be deposited in the hair => cc no melanin production

Question 25

Dihybrid cross affected by epistasis ratio:

Answer 25

9 black : 3 brown : 4 white

Question 26

Epistasis example in humans:

Answer 26

Albinism (autosomal recessive): albinism allele inhibits the expression of genes responsible for melanin production - due to absence / defect of tyrosinase (enzyme responsible f/ production of melanin)

Question 27

Polygenic inheritance -

Answer 27

2 or more genes control the expression of a single phenotype => additive effects; it is usually characterised by quantitative variation

Question 28

Quantitative characters:

Answer 28

characters that vary in the population along a continuum (continuous variation) ex: *height or skin colour in humans*

Question 29

Skin pigmentation is inherited by

Answer 29

at least **3 separately inherited genes** (6 alleles)

Question 30

External and internal conditions that can affect some phenotypes:

Answer 30

- temperature (temperature-dependent tyrosinase in Siamese cats) - chemicals - nutritional habits

Question 31

Norm of reaction:

Answer 31

the phenotypic range of genotype influenced by environment => broadest f/ polygenic characters

Question 32

Humans are not good subjects for genetic research (3)

Answer 32

– Generation time is too long – Parents produce relatively few offspring – Breeding experiments are unacceptable

Question 33

Examples of **autosomal recessive disorders** (4):

Answer 33

cystic fibrosis sickle cell anaemia thalassaemia albinism

Question 34

sickle cell anaemia

Answer 34

- The disease is caused by the substitution of a single amino acid (Glu→ Val) in the haemoglobin protein in red blood cells - **Symptoms**: physical weakness, pain, organ damage, and even paralysis - Heterozygotes are usually healthy but may suffer some symptoms; also, they are less susceptible to the malaria parasite: there was a survival advantage for being heterozygous in the Mediterranean / subtropical regions (*natural selection principles*)

Question 35

Cystic fibrosis

Answer 35

- The **cystic fibrosis allele** results in defective or absent *chloride transport channels in plasma membranes* - Characterised by scarring (fibrosis) and cyst formation within the pancreas. ***Symptoms*** include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Question 36

Why are autosomal dominant disorders less common than autosomal recessive disorders (2):

Answer 36

-The affected individuals usually do not survive until adulthood to produce offspring (**EXCEPTION**: disorders that express symptoms late in life (late onset of symptoms), ex: *Huntington disease*) - Even if they survive, the disorder presents in the phenotype (=> against the natural selection principles)

Question 37

Examples of **autosomal dominant disorders** (2):

Answer 37

Huntington’s disease: neurodegenerative disorder Achondroplasia: a form of dwarfism caused by a rare dominant allele

Question 38

Examples of **X-linked recessive disorders** (3):

Answer 38

haemophilia colour blindness Duchenne muscular dystrophy

Question 39

X-linked dominant disorders inheritance IF FATHER is affected and mother is not:

Answer 39

to ALL the daughters and none of the sons

Question 39

Examples of **X-linked dominant disorders** (2):

Answer 39

Vitamin D resistant (hypophosphataemic) rickets Alport syndrome

Question 40

X-linked dominant disorders inheritance IF MOTHER is affected and father is not:

Answer 40

to 50% of the children regardless of sex

Question 41

Prenatal screening:

Answer 41

- testing for diseases or conditions in a fetus or embryo before it is born to detect birth defects such as: 1. **chromosomal abnormalities** (ex: *Down syndrome*) by **karyotyping** 2. **genetic disorders** (ex: *sickle cell anaemia, thalassemia, cystic fibrosis, muscular dystrophy*, etc) by **genetic testing**

Question 42

Prenatal screening: Fetal Testing

Answer 42

- **Amniocentesis**: the amniotic fluid is removed and tested - **Chorionic villus sampling (CVS)**: a sample of the placenta is removed and tested - **Non-Invasive Prenatal Diagnosis (NIPD)**: a sample of mother’s blood, which contains some fetal cells, is removed and tested (new genetic test) All 3 techniques are followed by karyotyping (or genetic testing) of fetal cells, which can reveal possible abnormalities Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

Question 43

Newborn Screening

Answer 43

Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals. ex: *phenylketonuria* - autosomal recessive, patient cannot metabolize the aminoacid phenylalanine => Accumulation in blood and urine => mental retardation. Early detection => treatment with diet low in phenylalanine