17 Inheritance Flashcards
DNA
A large molecule made from bases, and which carries genetic information in the form of genes.
Deoxyribonucleic acid
Chromosome
The DNA in the nucleus of a cell is coiled and packed to form a thread-like structure called a chromosome
Genes
A length of DNA that codes for a protein.
Allele
An alternative version of a gene. A section of DNA that codes for a particular protein.
Haploid and diploid
a gamete has a haploid nucleus – it contains a single set of unpaired chromosomes
a body cell has a diploid nucleus – it contains two sets of chromosomes.
The DNA of human body cells contains two sex chromosomes:
males have one X chromosome and one Y chromosome, XY
females have two X chromosomes, XX.
a sperm cell (the male gamete) has one X chromosome or one Y chromosome
an egg cell (the female gamete) contains one X chromosome.
DNA consists of
DNA consists of two strands that coil together to form a double helix (two spirals that wrap around each other). Each strand contains chemicals called bases.
There are four bases, A, T, C and G. They always pair up this way:
A with T
C with G.
These are called complementary base pairs
Why sequence matters
The sequence (order) of bases differs from gene to gene. The sequence of bases in a gene is its genetic code: it determines the order in which amino acids are joined together to make a specific protein
Gene expression
Different sequences produce different proteins. These proteins can include:
enzymes
antibodies
receptors for neurotransmitters
Every cell in the body has a copy of every gene, but different genes are expressed at different stages in the life of the cell, and some genes may not be expressed at all.
Protein synthesis
A DNA molecule in the nucleus unzips, exposing the bases in a gene.
An mRNA molecule is formed using the exposed bases of the DNA strand as a template. The bases in the mRNA molecule pair with the complementary bases in the DNA, so the mRNA molecule carries a copy of the gene.
The mRNA molecule moves out of the nucleus into the cytoplasm.
A ribosome reads the mRNA code and joins amino acids together to form a protein, following the specific sequence of bases in the mRNA.
Mitosis
Mitosis is a type of nuclear division that gives rise to genetically identical cells. The cells produced by nuclear division are called daughter cells (even if they are not part of a female).
The role of mitosis
When a zygote is formed during the fertilisation of an egg cell by a sperm cell, it consists of just one cell. The zygote will grow and develop as more cells are produced through mitosis.
After an injury, new cells are needed to repair the damaged tissues. Mitosis is responsible for the repair of damaged tissues and the replacement of cells.
It is also involved in asexual reproduction.
Mitosis and cell division
In preparation for mitosis, each chromosome is replicated exactly so that there are two copies of each chromosome when the cell is about to divide.
This is when they become visible under the microscope as X-shaped structures because the two copies are joined together near the middle.
When the nucleus divides into two, one copy of each chromosome goes into each new cell during cell division.
This keeps the number of chromosomes in the two daughter cells the same as in the original cell.
Stem cells
unspecialised cells that divide by mitosis, producing daughter cells that can become specialised for specific functions.
Stem cells are found at sites such as embryos and umbilical cords, and adult bone marrow (the spongy part inside bones).
These cells can differentiate so they become specialised for a particular function.
This allows them to form various tissues, organs and organ systems.
These cells are still genetically identical in an individual person, but their genes are expressed differently
Miosis
Meiosis is a type of nuclear division that gives rise to cells that are genetically different
Meiosis is involved in the production of gametes. In humans:
male gametes (sperm cells) are produced in the testes
female gametes (egg cells) are found in the ovaries and mature there.
Meiosis occurs in the production of gametes of all organisms that reproduce sexually, including flowering plants.
Meiosis and variation
Like mitosis, each chromosome is exactly duplicated before meiosis begins
In meiosis, the number of chromosomes is halved, so it is called a reduction division. Each cell will have one chromosome of each pair.
In meiosis, a diploid cell (with a nucleus containing two sets of chromosomes ) becomes four haploid cells (each with a nucleus containing a single set of unpaired chromosomes).
The haploid cells formed are genetically different from each other, and from the original diploid cell. This variation happens in two ways.
A process called crossing over occurs in an early stage of meiosis. The chromosomes in a pair of chromosomes swap parts with each other Two new chromosomes form, each of which is different from the two original chromosomes.
In later stages of meiosis, the cell divides twice to form four daughter cells
Inheritance
Is the transmission of genetic information from generation to generation. An organism that receives genetic information from its parent (if asexual reproduction has occurred) or its parents (if sexual reproduction has occurred) is said to have inherited this information.
Homozygous and heterozygous
homozygous if there are two identical alleles
heterozygous if there are two different alleles.
Phenotype and genotype
The observable features of an organism.
An organism’s genotype is all the alleles present in that organism.
Monohybrid cross
Breeding two individuals that differ in their alleles for a particular gene.
dominant and recessive genes
A dominant allele is always expressed if it is present
A recessive allele is only expressed if the dominant allele is not present.
In practice, this means that a dominant allele is expressed if one or two copies are present, but a recessive allele is only expressed if two copies are present.
genetic symbol
Use the same letter
Y(capital) represents the dominant allele
y(lowercase) represents the recessive allele.
Codominace
A situation in which two different alleles for a gene influence the phenotype to the same extent, for example in human ABO blood groups.
Blood group
ABO blood groups
People can belong to blood group A, B, AB or O.
Genotype Blood group
I A I A or I A I O: A
I B I B or 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).
