17. Inheritance Flashcards
DNA - What is it
A large molecule made from bases, and which carries genetic information in the form of genes.
Two strands coiling together to form a double helix. (each strand contains a sequences of bases)
(one molecule)
–>
DNA is tightly coiled + contains large numbers of atoms
Chromosomes
Thread-like structures of DNA, carrying genetic information in the form of genes.
A structure containing tightly coiled DNA which is found in the nucleus
Gene
A length of DNA that codes for a particular protein.
Defines the genetic information whcih DNA carries.
Chromosomes + Genes
Every chromosome PAIR contains different types of genes (coding for different proteins)
In a chromosome pair, both chromsome contain the same gene (but they either have the same or different allele/ version of the gene)
Haploid
Single set of chromosomes (a type of nucleus), eg. gamete
23 unpaired chromosomes
Diploid
Double set of chromosomes (a type of nucleus), eg. body cells.
23 pairs of chromosomes
Sex chromosomes
Two out of 46 chromosomes are sex chromosomes (determine sex of individual)
Either X or Y chromosomes…
Females - XX
Males - XY
Sex chromosomes in gametes
The egg cell ALWAYS contains one X chromosome (inherited from mother)
The sperm cell contains EITHER an X chromosome or a Y chromsome (inherited from father)
—> Therefore sex of child is dependent on father (x –> girl, y –> boy)
Base sequence in DNA
Each strand contains a sequences of bases.
Base sequence determines genetic code (order of amino acids)
four bases = A, T, C, G
Complementary base pairs –>
A + T
C + G
What happens when a gene is expressed?
When a gene is being used in a cell to make a protein, it’s described as being expressed.
Some genes are never used in a cell, other are at different stages.
(eg. insulin production gene is only expressed in pancreas)
Stages of protein synthesis
- A DNA molecule in the nucleus unzips → exposing the bases in a gene.
- Using the exposed bases of the DNA strand as a template → an mRNA molecule is formed.
- This happens through the bases in the mRNA molecule pairing with the complementary bases in the DNA. → Therefore the mRNA carries a copy of the gene.
- The mRNA molecule then moves out of the nucleus and into the cytoplasm.
- A ribosome reads the mRNA code and then uses the code to join amino acids together to form a protein.
Why is mRNA used to transport DNA?
DNA molecules are too large, mRNA only copies the gene needed.
mRNA structure vs DNA structure
mRNA = single stranded + base T is replaced by base U
Mitosis
A form of nuclear division that gives rise to genetically identical cells.
Stages of mitosis
- Chromsomes duplicate (they look like x’s as they duplicate)
- Nucleus of the cell divdes
- Chromosome ‘x’s’ (duplicates) divide into single strands of chromosomes, so each cell has 46 each.
1 cell → 2 cells → 4 cells → 8 cells
When does mitosis occur?
Zygote division
Repairing damaged tissues/ replacement of cells
Asexual reproduction
Stem cells
An unspecialised cell that can develop into various differentiated cell types.
The cells are still genetically identical in an individual person, but their genes are expressed differently.
Where are stem cells found?
Embryos
Umbilical cords
Adult bone marrow
Meiosis
A type of nuclear division that gives rise to cells that are genetically different.
Involved in production of gametes (in all organisms that sexually reproduce, includign flowering plants)
Stages of meiosis
- A diploid cell duplicates its chromosomes.
- Divides into two diploid cells.
- Then each diploid cell halves, forming in total four haploid cells.
(all haploid cells are genetically different from each other)
How does variation occur during meiosis?
As the chromosomes duplicate (look like x’s), the chromosomes in a pair swap parts with each other (aka they ‘crossover’)
The crossed over chromosomes then divide to form haploid cells.
eg.
(X, X) (Chromosome pair in duplication process)
(X, X) (crossover)
(X) (X) (chromosome pair divided into 2 haploids)
(l) (l) (duplicated chromosome divided into 1 unpaired chromosome –> haploid)
Inheritance (def + types of inheritance)
The transmission of genetic information from generation to generation.
Genotypic
Sex
Blood group
Natural hair colour
Phenotypic
Language
Both
Height + Body mass
Homozygous
Two identical alleles in a chromosome pair
Heterozygous
Two different alleles in the chromosome pair.
Allele
An alternative version of a gene
Genotype
An organism’s genotype is all the alleles present in that organism.
(The genetic make-up in terms of the versions of genes in contains)
Phenotype
The observable features of an organism.
(Some aspects of the phenotype is a result of teh genotype, eg. eye colour)
Homozygous green seed * Homozygous green seed (pea plant example)
–> Only one possibility of genes can be inherited.
(green seeds because both parents only have the green seed gene)
Heterozygous seed colour * Homozygous green seed
The offspring will inherit an allele for green seeds from their homozygous parent.
However, they can inherit either the green or yellow seed allele from their heterozygous parent.
Dominant
When present the dominant allele will always be expressed.
Recessive
When present the recessive allele will only be expressed if the dominant allele is not present.
Dominant/ recessive combinations
Dominant + Recessive = Dominant
Recessive + Dominant = Dominant
Dominant + Dominant = Dominant
Recessive + Recessive = Recessive
Genetic symbols
A dominant allele is shown by an uppercase letter
A recessive allele for the same gene is shown by the same letter but in lowercase.
Codominance
In some cases, two different alleles for a characteristic influence the phenotype to the same extent.
Eg. Blood group
Blood group - Codominance
People can belong to blood group A, B, AB or O.
The gene involved is shown by an uppercase letter, I and then a subscript uppercase letter.
Possible genotypes…
I^A + I^A –> A
I^A + I^O –> A
I^B + I^B –> B
I^B + I^O –> B
I^A + I^B –> AB
I^O + I^O –> O
Alleles I^A and I^B are both dominant to allele I^O
Alleles I^A and I^B are codominant to each other – a person inheriting both alleles is blood group AB (not A or B).
Sex linkage
Sex in humans is determined by the two sex chromosomes, X and Y.
Males are XY and females are XX.
Sex chromosomes don’t just determine the sex of an organism.
–> They also carry certain genes which determine other characteristics.
Eg. Haemophilia + red-green colour blindness are recessibe genes carried on X chromosome.
Y vs X chromosome
The X chromosome carries many more genes than the Y chromosome because it is much longer.
Many genes found on the X chromosome are not found on the Y chromosome.
Why is it more common for men to be colourblind / haemophiliacs
The gene is recessive and carried on the X chrosomes
Women need two recessive genes (X^r + X^r)
However, men only need one recessive gene (X^r + Y)
–> Making it more likely.
