Ch 4: Mendellian Genetics Flashcards
What is heredity?
the study of inheritance; the genetic trasnmission of characteristics from one generation to another
What are Mendel’s laws?
- principle of segregation: the two members of a gene pair (alleles) segregate from each other in the formation of gametes. Half the gametes carry one allele and the other half carry the other allele
- principle of independent assortment: genes for different traits assort independently of one another in the formation of gametes
What is a genotype and phenotype?
genotype: genetic composition of an organism, a genotype may be homozygous or heterozygous and the alleles interact to express phenotypes that are dominant, recessive, partially dominant or codominant
phenotype: the way genes are expressed in the outward appearance of an organism
What are the types of genotype?
- dominant: phenotype that requires only one copy of its allele in an individual to be expressed
- recessive: phenotype that requires two copies of its allele in an individual to be expressed
- codominant: a state in which both alleles of a heterozygous individual are fully expressed in the phenotype i.e. snap dragons, a purebred red and white parent produce a pink offspring
- partially dominant: (or incomplete) the state in which a heterozygous individual has a phenotype that is intermediate between those of corresponding homozygous individuals i.e. CWCW + CR CR= CWCR (red/white mix - dappled)
What is a monohybrid cross?
cross that involves one pair of contrasting phenotypes
- the actualy ratios obtained in genetic crosses only apprroximate to the expected ratios due to random fusion of gametes as well as fatality of gametes (failure to develop, link or give rise for a zygote)
INSERT PHOTOWhat is a test cross and give an example?
a cross using an organism with a recessive phenotype to determine the unknown genotype of an organism with a dominant phenotype
What are deadly combinations and lethal phenotypes and give an example?
homozygous lethal phenotype: a phenotype that arises from a homozygous recessive gentoype leading to the premature death of an organism, most common lethal phenotype
- certain dominant phenotypes have become so important for normal development that an individual homozygous gor a mutant recessive cannot survive (can be perpetuated in a heterozygous population)
Huntington’s disease
- dominant lethal phenotype characterised by the deterioration of the nervous system
- individuals with a single copy of the malfunctioning allele always develop the disease
- such phenotypes usually persist in a population if they cause death after reproductive age
What are multiple alleles for one gene and give an example?
when 3 or more alleles of a gene exist among the members of population
Human blood group type
codominant: IA and IB produce molecular markers on red blood cells
recessive: i produces no molecular markers
Blood type phenotype -
- A : IA IA , IAi
- B : IBIB , IBi
- AB: IAIB
- O: ii
Describe inheritance of multiple autosomal genes and dihybrid crosses
dihybrid inheritance: inheritance of two pairs of contrasting characteristics
- inheritance for 2 unlinked autosomal genes can be analysed with a dihybrid cross. If P is pure bred, F2 ratio is 9:3:3:1
- alleles for different characteristics (i.e. height and flower) segregate and assort independently because they are carried on separate chromosomes, which themselves segregate and assort independently in meiosis
- the same phenotype may result from several different genotypes i.e.tall purple plant may be TTPP, TTPp, TtPP, TtPp
- to work out the genotype of a tall purple plant, cross it with a short white one, if it is
- TtPp - 4 types of offspring in equal numbers (tall purple/white, short purple/white)
- TTPp - tall purple and tall white in equal numbers
- TtPP - tall purple and short purple in equal numbers
Define polygenic inheritance, polygenes and polygenic characteristic. Give an example of polygenice inheritance
polygenic inheritance: trasnmission between generations of characteristics that are controlled by polygenes
polygenes: a gene for which the alleles have a small additive effect on a phenotype; many polygenes contribute to continuous variation in a phenotype
polygenic characteristic: characteristic controlled by the alleles of 2 or more genes interacting with each other
Human height
- humans have a range of heights with a smooth gradation from one extreme to another
- 200 genes have been identified as contributing to height
- majority of genes have a modest effect, while some have great influence i.e. GH1, growth hormone
- short stature can be caused by mutations that abolish the contribution of a gene encoding GH1
- gigentism can be caused by cancers to the pituitary gland when GH1 is produced, overstimulating production
Differentiate continuous and discontinuous variation
continuous variation: a variation in characterisitcs that shows a small range of different phenotypes
discontinuous variation: a variation in a characteristic that shows two or just a few clearly distinct phenotypes, occurs when only one gene is involved and results in a small no. of phenotypes (i.e. peaplants with either P or W flowers, but no other colours)
What are the regions of sex chromosomes?
- differential: regions unique to each chromosome, genes within these show sex linked inheritance patterns, sex-linked inheritance can be detected as phenotypes that segregate differently between males and females (uneven phenotypic ratios between sexes)
- in males, hemizygous as there is only a single copy of each of them
- homologous: regions that the 2 chromosomes share in common
What are features of the diffferent types of sex-linked inheritance_?_
X-linked recessive
- recessive phenotype determined by an allele on the X chromosome
- more common in males as males only need one copy of the affected allele (no male carriers)
- females will only express the phenotype when both X chromosomes have the affected allele
- males affected if mother affected, daughters carrier if father affected (or affected if morther affected/carrier)
X-linked dominant
- show up in all affected females and males
- son’s will not receive affected allele from affected father, heterozygous female expected to pass on the allele to 50% of offspring
Y-linked
- exclusively male
- most commone phenotype associated with this is Maleness in humans, determined by the SRY gene on the Y chromosome
